[mlir][llvm] Remove the metadata op

This revision removes the metadata op, that to the best of our
knowledge, has no more uses after switching to a purely attribute based
metadata representation:
https://reviews.llvm.org/D155444
https://reviews.llvm.org/D155285
https://reviews.llvm.org/D155159
These changes got unlocked after landing distinct attribute support:
https://reviews.llvm.org/D153360,
which enables modeling distinct metadata using attributes. As a result,
all metadata kinds are now represented using attributes. Previously,
there has been a mix of attribute and op based representations.

Having attribute only metadata makes it possible to update the metadata
in-parallel, while updating the global metadata operation has been
a sequential process. The LLVM Dialect inliner already benefits from
this change and now creates new alias scopes and domains during
inlining rather than dropping the no alias information:
https://reviews.llvm.org/D155712

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D156217