isValidResourceIndex in Program.h is unused.

BUG=angleproject:1920

Change-Id: Ide097348fac7ed97a0e07f3d97d7944ac997f71d
Reviewed-on: https://chromium-review.googlesource.com/474229
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>