Remove obsolete support for sk_InColor in GrSkSLFP

There is no way to get a variable with that builtin identifier (it's
going away entirely soon), so no need for the protocol to send it to
the FP.

Bug: skia:10619
Change-Id: Icd1744d8573ded2a6fdf3c1901f46df0c91aab14
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310760
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed