Remove non-conic stroke shader specializations

Every stroke shader just has conic branches now. The conic/cubic math
is mostly unified anyway, so a couple small branches shouldn't affect
perf.

Bug: skia:10419
Change-Id: I7690e54e61413902dd9836da532faf6383fb73b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/412456
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
7 files changed