feat(update): remove create or update

create or update introduces a new pattern in addition
to the existing flow of:

1. getting a resource.
2. if NOT_FOUND, send a create.
3. if found, send an update.

In the future, this could be re-added as an Apply method,
which would map to the appropriate REST method of PUT.

aep/general/0134/aep.md.j2

@@ -170,51 +170,6 @@ If a rating were set on a book and the existing `PUT` request were executed, it
 would wipe out the book's rating. In essence, a `PUT` request unintentionally
 would wipe out data because the previous version did not know about it.
 
-### Create or Update
-
-If the service uses client-assigned resource paths, `Update` methods **may**
-expose a `bool allow_missing` field, which will cause the method to succeed in
-the event that the user attempts to update a resource that is not present (and
-will create the resource in the process).
-
-{% tab proto %}
-
-```proto
-message UpdateBookRequest {
-  // The book to update.
-  //
-  // The book's `path` field is used to identify the book to be updated.
-  // Format: publishers/{publisher}/books/{book}
-  Book book = 1 [(google.api.field_behavior) = REQUIRED];
-
-  // The list of fields to be updated.
-  google.protobuf.FieldMask update_mask = 2;
-
-  // If set to true, and the book is not found, a new book will be created.
-  // In this situation, `update_mask` is ignored.
-  bool allow_missing = 3;
-}
-```
-
-{% tab oas %}
-
-**Note:** OAS example not yet written.
-
-{% endtabs %}
-
-More specifically, the `allow_missing` flag triggers the following behavior:
-
-- If the method call is on a resource that does not exist, the resource is
-  created. All fields are applied regardless of any provided field mask.
-  - However, if any required fields are missing or fields have invalid values,
-    an `INVALID_ARGUMENT` error is returned.
-- If the method call is on a resource that already exists, and all fields
-  match, the existing resource is returned unchanged.
-- If the method call is on a resource that already exists, only fields declared
-  in the field mask are updated.
-
-The user **must** have the update permissions to call `Update` even with
-`allow_missing` set to `true`.
 
 ### Etags