With PBOs being exposed through an extension in ES2 now, this check is
uneccessary.

Change-Id: I609a4bcd944194d104e0774e5f6ea34e072c8d9c
Reviewed-on: https://chromium-review.googlesource.com/201320Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>