Disable UNIMPLEMENTED in ProgramGL::validateSamplers.
Disable UNIMPLEMENTED in ProgramGL::validateSamplers.

Removing this allows many angle end2end tests to run without hitting any
UNIMPLEMENTEDs.

BUG=angleproject:882

Change-Id: Idb55fae7b6fc7b23bc85d0bf376499f28669d771
Reviewed-on: https://chromium-review.googlesource.com/263250
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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