Remove the last use of fragmentProcessor field access in .fp files

This is a complex feature that was propping up this one use-case. After
this CL, it can be removed.

Change-Id: Ia391565daddeba46adfb325980d10ad653e269c8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375019
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
6 files changed