Metal: remove unused variables

It's not clear why this isn't failing on the bots, but it throws a
warning.

Change-Id: Id5eb88e6620efb5bbfcdb9bba83a7542e0d0e2a8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439165
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
1 file changed