Remove [[maybe_unused]] workaround in MetalCodeGen.

This effectively rolls back http://review.skia.org/538045.

Change-Id: I9c457a6c9c9ba98d4c2e3a6af03c5dd80c76beb9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/540082
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed