CSS shader has been removed from spec and chrome code base. Remove the
code in ANGLE.

BUG=chromium:233383

Change-Id: I93a35437f540e51ce7af9d49f21ca60d7c0b156a
Reviewed-on: https://chromium-review.googlesource.com/378739Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>