Address couple trim warnings

[manodasanW]

• The delegate in IDisposable seems to cause trim warnings when used in RegisterCustomAbiTypeMappingNoLock. It doesn't seem to be used, so removed it and confirmed the warnings went away.
• Improved trim warning for C#/WinRT authoring support.

Fix #1269

[jkoritzinsky]

Are you sure this addresses the trim warning I mentioned? I could have sworn the issue was the typeof(EventHandler<>) that was causing the issue.

[manodasanW]

Are you sure this addresses the trim warning I mentioned? I could have sworn the issue was the typeof(EventHandler<>) that was causing the issue.

I confirmed with a repro I got and then updated it to 2.0.1-prerelease.221115.11 and no longer saw those warnings. We can use the same build to confirm with @Sergio0694 's repro. If you look at it, you will note that typeof(EventHandler<>) is passed for publicType which has no DynamicallyAccessedMembers markings. It is the abiType which does which is getting our ABI version of that type. The only warning left I see is for GetAuthoringMetadataType which I put a better warning for now.

        private static void RegisterCustomAbiTypeMappingNoLock(
            Type publicType,
#if NET
            [DynamicallyAccessedMembers(
                DynamicallyAccessedMemberTypes.PublicMethods |
                DynamicallyAccessedMemberTypes.NonPublicMethods |
                DynamicallyAccessedMemberTypes.PublicNestedTypes |
                DynamicallyAccessedMemberTypes.PublicFields)]
#endif
            Type abiType, 
            string winrtTypeName,
            bool isRuntimeClass = false)

[jkoritzinsky]

It's possible that I was looking at the wrong thing. Looks like you fixed it!