remove Gauss option from SkGaussFilter

Unused except in unit tests.

Change-Id: I07bb70515a383898ad59c0fa918461052b582e35
Reviewed-on: https://skia-review.googlesource.com/c/168860
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed