Remove unused SkSafe_math.h

This file has not been included in a while, so it seems unnecessary.

Change-Id: I72251956146309a0d19801e0b3348c1ca286fa3b
Bug: 666707
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/630546
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
5 files changed