Remove SkSLCPP.h, utilities that were only used by .fp generated code

Change-Id: I00f60e04a481613656760d71979540a75c353936
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/426439
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
4 files changed