Revert "[Sema] Warn on attribute nothrow conflicting with language specifiers"

This reverts r314461.

It is warning on user code that uses END_COM_MAP(), which expands to
declare QueryInterface with conflicting exception specifers. I've spent
a while trying to understand why, but haven't been able to extract a
reduced test case. Let's revert and I'll keep trying.

llvm-svn: 314689