This check was not used. It applied only to rendering Feedback Loops.
The behaviour is undefined in non-WebGL and for WebGL we have a
separate validation check.

Also update the feedback loop tests to ignore the current GL states.
This change is based on feedback from the OpenGL ES working group.

Bug: angleproject:2763
Bug: chromium:763695
Change-Id: I9882b4f9af2d43fc7b5604ff36dadcc79dfd378f
Reviewed-on: https://chromium-review.googlesource.com/1228373
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>