The Qualcomm bug workaround is changed such that clears still go through
the render pass loadOp, but the render pass is immediately closed.  This
allows us to remove a few fallback methods.

Bug: angleproject:2361
Change-Id: I24c3884a183f8bb40673e922773f70faffad848f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1545524Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>