Drop unused shader support for multisample interpolation

Bug: skia:
Change-Id: Ie19f13877bf6828ab94ffbb20e6f74e0e376f6cb
Reviewed-on: https://skia-review.googlesource.com/107359
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
6 files changed