GenAPI emits nullable attributes

[ericstj]

Describe the bug

Now that the Nullable attributes are defined publicly in the framework instead of internally in the user assembly they are no longer excluded by the visibility filter. We shouldn't emit these attributes, but instead emit the nullable annotations on the C# syntax.

See #33841 for examples of the unwanted nullable attribu tes.

To Reproduce

Compile an assembly that uses nullable annotations targeting a recent build of .NET 8.0 with dotnet/runtime@d39e0c4, then run genapi on the resulting assembly.

Expect: public API that matches what was defined.
Actual: public API with explicitly references to NullableAttribute, NullableContext, etc.

Exceptions (if any)

None

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