Remove SkRGBA4f pinning functions

Bug: skia:
Change-Id: I48ba39465a160e73502ee38d90062d955da6e8e6
Reviewed-on: https://skia-review.googlesource.com/c/167140
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed