Consider Removing Support for [[suppress-warning:invalid-metadata]]

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Currently There are 3 categories of warnings that can be suppressed:
Deprecated, InvalidMetadata, and InvalidComment.

There's an obvious use case for suppressing deprecation warnings (you're using an old API and can't update to the new one yet).
I also think there's a case for suppressing comment warnings (you're using doxygen tags that the Slice compiler isn't aware of, but that are still valid).

But there isn't a good case for ignoring bad metadata, especially now that the metadata validation is accurate and robust.
I think that issuing warnings is correct, we shouldn't halt compilation and fail to build over metadata issues.
But these also are not the kinds of things you should just ignore and leave unfixed.

Therefore, I propose dropping support for suppressing InvalidMetadata warnings.

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Note that suppress warnings also has support for an All argument, which of course ignores all warnings.
I think that this behavior should be preserved. So if you suppress All, this would still include InvalidMetadata warnings,
despite there being no other way to do suppress invalid metadata.

This is reasonable because if a user is suppressing all warnings, they're clearly doing something strange, and do not want the Slice compiler bothering them about anything. And it would be very strange to see the compiler issuing warnings after you told it to suppress all of them.

Note that we already have some "misc" warnings that can only be ignoring by the All option.
For example, ice2slice issues warnings for consts (since there's no way to map them into new Slice), and there is no category of warnings you can use to ignore these warnings.