The prefix is unnecessary now that user-defined names are prefixed in
both GLSL and HLSL output. Removing the prefix makes compiler output
a bit simpler to read.

BUG=angleproject:2038
TEST=angle_unittests

Change-Id: I9ffc508f50d6146a2d85798875c88e2c385b83fe
Reviewed-on: https://chromium-review.googlesource.com/508730Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>