The division between 2- and 3-parameter built-ins is superfluous and
can be removed. Also folding support for ESSL 3.00 is complete, so the
comments about needing to add folding support for more functions can
be removed.

BUG=angleproject:1730
TEST=angle_unittests

Change-Id: I5ca93a957a98a7740eca506e44af48e7b4b83bad
Reviewed-on: https://chromium-review.googlesource.com/431108Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>