cogl: Remove viewport scissor workaround
This is effectively a revert of:
commit 6cfc93f26f64c506922bf119d5079d83de7543d2
Author: Robert Bragg <robert@linux.intel.com>
Date: Tue Oct 2 11:44:00 2012 +0100
clip-stack: workaround intel gen6 viewport clip bug
It's been over six years, if this bug is still present we should just fix Mesa already.