remove SkArithmeticMode.h empty header

needs associated fix in critique to land, removing a reference to the header.

BUG=skia:

Change-Id: I55c919e0cca801ea424641253d3fb406cb73fd12
Reviewed-on: https://skia-review.googlesource.com/7729
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed