This reverts commit 27906e9c.

Reason for revert: Causes failures in mac_optional_gpu_tests_rel

Original change's description:
> GL: Cache the results of glGetString
>
> To reduce the amount of queries to the driver, we can cache the
> results of glGetString. On Mac, we need to invalidate this cache
> on GPU switch.
>
> Bug: chromium:1173672
> Change-Id: I039172068aec35034a87881a8804f52c080ce4ce
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2676882
> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Geoff Lang <geofflang@chromium.org>

TBR=geofflang@chromium.org,jonahr@google.com,jmadill@chromium.org

Change-Id: I5799c5d287dd01b946446bd66d4c89aef6756d3a
Bug: chromium:1173672
Bug: angleproject:5639
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2690145Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>