rip out gb1 and srgb feature flag

fixes CNVS-13293

test plan:
 - regression on gb2 and srgb
   to make sure they both still work properly

Change-Id: I4ec554589b6a77a113d2faf46e7b530faa2d535f
Reviewed-on: https://gerrit.instructure.com/35501
Product-Review: Dana Danger <dana@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>