remove unused ConvertRadiusToSigma from SkBlurMaskFilter

Bug: skia:
Change-Id: I1726f22fc40ad61b1b0485bcda6d383614da1fdb
Reviewed-on: https://skia-review.googlesource.com/113463
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
8 files changed