Remove instanced flush WAR for Skylake

There don't seem to be any more rendering artifacts without the WAR.
Maybe the drivers got updated, or maybe something in our API usage
changed.

TBR=bsalomon@google.com

Bug: skia:8566
Change-Id: Id82588f445fd61a44e77b988c31fb7f46848fffa
Reviewed-on: https://skia-review.googlesource.com/c/173257
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed