Remove these needless checks as GetActiveShaderVariables in Shader.cpp has
filtered them already.

BUG=angleproject:1920

Change-Id: Ife85614a8f636768dd585a827bca81e2f15009c8
Reviewed-on: https://chromium-review.googlesource.com/454129
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>