Remove WinRT specific code from EventSource.

[eerhardt]

Since we no longer have WinRT specific code in the runtime, we don't need these build defines in the EventSource code. No one is setting these defines, and it is confusing when looking at the code what types are public and what aren't.

Follow up to #36715

cc @brianrob

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[sywhang]

LGTM.

[brianrob]

You will also want to get rid of the GetMetadata method and all references to it: https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventSource.cs#L712.

It was added to provide a mechanism for EventSource to fetch metadata that was produced as part of the build.

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