Remove SkSL "Generic" programs

These were arbitrary signature functions that only worked with the SkVM
backend. With particles gone, they're no longer used.

Bug: skia:13677
Change-Id: Iebdbec8a5a2d663247556f72800b784117778257
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/648740
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
9 files changed