Metal: Remove unused variable

Bug: skia:12086
Change-Id: I5564e4981575cab546e4d2d0470656b0aff204c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443877
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