This helper was redundant with the image utils library. Switch the test
to use the existing methods. Doesn't affect test correctness.

This removes more 'configless' tests from angle_end2end_tests.

Bug: angleproject:3393
Change-Id: I49cfbd95c3713b2b4b25c4716a7a279d84ddf994
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1591426Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>