Remove layout(override_coverage)

This was only used in the context of sk_SampleMask, which was removed
recently.

Change-Id: Id70d7af8b3a100ff157c2984bad4131f1b92f317
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/410056
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed