Bug 1363819 - Stop running Android 4.3 mochitest-webgl; r=jmaher

Android mochitest-webgl tests have been hidden on treeherder for months
and 8 of the 10 chunks are currently perma-fail.

taskcluster/ci/test/test-sets.yml

@@ -234,7 +234,6 @@ android-common-tests:
     - mochitest-clipboard
     - mochitest-gpu
     - mochitest-media
-    - mochitest-webgl
     - reftest
     - xpcshell