Remove attributes that aren't yet available in C# 8 (#1218)

* Remove attributes

The `MemberNotNull` and `MemberNotNullWhen` attributes weren't available in C# 8. They were added in C# 9.

* fix build warnings

Missed two notes on these attributes

standard/attributes.md

@@ -877,8 +877,6 @@ The code-analysis attributes are declared in namespace `System.Diagnostics.CodeA
 `MaybeNullWhen` ([§22.5.7.7](attributes.md#22577-the-maybenullwhen-attribute))  | A non-nullable argument may be null when the method returns the specified `bool` value.
 `NotNullWhen` ([§22.5.7.12](attributes.md#225712-the-notnullwhen-attribute))  | A nullable argument won’t be null when the method returns the specified `bool` value.
 `NotNullIfNotNull` ([§22.5.7.11](attributes.md#225711-the-notnullifnotnull-attribute))  | A return value isn’t null if the argument for the specified parameter isn’t null.
-`MemberNotNull` ([§22.5.7.8](attributes.md#22578-the-membernotnull-attribute))  | The listed member won’t be null when the method returns.
-`MemberNotNullWhen` ([§22.5.7.9](attributes.md#22579-the-membernotnullwhen-attribute))  | The listed member won’t be null when the method returns the specified `bool` value.
 `DoesNotReturn` ([§22.5.7.4](attributes.md#22574-the-doesnotreturn-attribute))  | This method never returns.
 `DoesNotReturnIf` ([§22.5.7.5](attributes.md#22575-the-doesnotreturnif-attribute))  | This method never returns if the associated `bool` parameter has the specified value.
 
@@ -1032,47 +1030,6 @@ Specifies that a non-nullable return value may be null.
 
 Specifies that a non-nullable argument may be `null` when the method returns the specified `bool` value. This is similar to the `MaybeNull` attribute ([§22.5.7.6](attributes.md#22576-the-maybenull-attribute)), but includes a parameter for the specified return value.
 
-#### 22.5.7.8 The MemberNotNull attribute
-
-Specifies that the given member won’t be `null` when the method returns.
-
-> *Example*: A helper method may include the `MemberNotNull` attribute to list any fields that are assigned to a non-null value in that method. A compiler that analyzes constructors to determine whether all non-nullable reference fields have been initialized may then use this attribute to discover which fields have been set by those helper methods. Consider the following example:
->
-> <!-- Example: {template:"standalone-lib", name:"MemberNotNullAttribute"} -->
-> ```csharp
-> #nullable enable
-> public class Container
-> {
->     private string _uniqueIdentifier; // must be initialized.
->     private string? _optionalMessage;
->
->     public Container()
->     {
->         Helper();
->     }
->
->     public Container(string message)
->     {
->         Helper();
->         _optionalMessage = message;
->     }
->
->     [MemberNotNull(nameof(_uniqueIdentifier))]
->     private void Helper()
->     {
->         _uniqueIdentifier = DateTime.Now.Ticks.ToString();
->     }
-> }
-> ```
->
-> Multiple field names may be given as arguments to the attribute’s constructor. *end example*
-
-#### 22.5.7.9 The MemberNotNullWhen attribute
-
-Specifies that the listed member won’t be `null` when the method returns the specified `bool` value.
-
-> *Example*: This attribute is like `MemberNotNull` ([§22.5.7.8](attributes.md#22578-the-membernotnull-attribute)) except that `MemberNotNullWhen` takes a `bool` argument. `MemberNotNullWhen` is intended for use in situations in which a helper method returns a `bool` indicating whether it initialized fields. *end example*
-
 #### 22.5.7.10 The NotNull attribute
 
 Specifies that a nullable value will never be `null` if the method returns (rather than throwing).

standard/standard-library.md

@@ -614,22 +614,6 @@ namespace System.Runtime.CompilerServices
         public MaybeNullWhenAttribute(bool returnValue) {}
     }
 
-    [System.AttributeUsage(System.AttributeTargets.Method | 
-    System.AttributeTargets.Property, AllowMultiple=true, Inherited=false)]
-    public sealed class MemberNotNullAttribute : Attribute
-    {
-        public MemberNotNullAttribute(string member) {}
-        public MemberNotNullAttribute(params string[] members) {}
-    }
-
-    [System.AttributeUsage(System.AttributeTargets.Method | 
-      System.AttributeTargets.Property, AllowMultiple=true, Inherited=false)]
-    public sealed class MemberNotNullWhenAttribute : Attribute
-    {
-        public MemberNotNullWhenAttribute(bool returnValue, string member) {}
-        public MemberNotNullWhenAttribute(bool returnValue, params string[] members) {}
-    }
-
     [System.AttributeUsage(System.AttributeTargets.Field |
       System.AttributeTargets.Parameter | System.AttributeTargets.Property | 
       System.AttributeTargets.ReturnValue, Inherited=false)]