Removing tracking of whether GP/FP coverage calculations are used by XP.

Removing tracking of whether GP/FP coverage calculations are used by XP.

Remove readsCoverage from GrPipelineOptimizations

Remove kNone from GrDefaultGeoProc::Coverage

Remove kIgnoreCoverage from GrXferProcessor::OptFlags

Remove GrPipeline::fIgnoresCoverage

Change-Id: I8f0c9337ee98b9c77af1e9a8a184519ce63822d4
Reviewed-on: https://skia-review.googlesource.com/6552
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>