remove mention of 16-bit shader procs

This is all dead.

Change-Id: Ifef5edb5c2402118cb289bbe31923f8de56bab23
Reviewed-on: https://skia-review.googlesource.com/c/171240
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed