rust/kernel: remove redundant Error::from_kernel_errno()

It turns out that we don't need an `Error` constructor that
checks the `errno` invariant at runtime. This is because we
trust return values originating from kernel C. And any return
values originating from Rust code can be constucted using
`Error::` constants, which also do not need a runtime check:

```rust
    return Err(Error::EBUSY);
```

If we find we do require this function in the future, we may
restore it simply by reverting this commit.

Signed-off-by: Sven Van Asbroeck <[email protected]>

rust/kernel/error.rs

@@ -60,25 +60,6 @@ impl Error {
     /// Bad file number.
     pub const EBADF: Self = Error(-(bindings::EBADF as i32));
 
-    /// Creates an [`Error`] from a kernel error code.
-    ///
-    /// It is a bug to pass an out-of-range `errno`. `EINVAL` would
-    /// be returned in such a case.
-    pub(crate) fn from_kernel_errno(errno: c_types::c_int) -> Error {
-        if errno < -(bindings::MAX_ERRNO as i32) || errno >= 0 {
-            // TODO: make it a `WARN_ONCE` once available.
-            crate::pr_warn!(
-                "attempted to create `Error` with out of range `errno`: {}",
-                errno
-            );
-            return Error::EINVAL;
-        }
-
-        // INVARIANT: the check above ensures the type invariant
-        // will hold.
-        Error(errno)
-    }
-
     /// Creates an [`Error`] from a kernel error code.
     ///
     /// # Safety