Adjust cache key for Metal persistent cache.

The Metal-specific parts of the ProgramDesc key aren't needed as we're
only storing source code at the moment.

Bug: skia:8243
Change-Id: I897f4a93bedafc954073466897605d753a40810e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/259416
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed