Simplify overdraw color filter a bit.

Nothing major, just removed an obsolete/meaningless `extern` and
de-cluttered the SkSL a bit.

Change-Id: I645403be6fbf0d023619a39f7f87a108d768503e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/665859
Reviewed-by: Arman Uguray <armansito@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
2 files changed