Revert "added support for SkSL unpremul function"

This reverts commit ffdc3e668061cff14ea55ba087e9105f2ff219fc.

Reason for revert: breaking Chrome roll

Original change's description:
> added support for SkSL unpremul function
> 
> Change-Id: I970f1ad0dd0859448c874498fe02342f8abc3aa3
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/242897
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

TBR=bsalomon@google.com,brianosman@google.com,ethannicholas@google.com

Change-Id: I72dacea82b1f4ea2aa6cd600122a4810657aa32e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243117
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
14 files changed