Fix is now upstreamed, we can remove the workaround.

This reverts commit 2ba626dc.

Original CL message:

> dEQP was inadvertendly requiring the presence of these extensions to
> even run the tests. There's a WIP fix to the dEQP tests and this CL
> implements that fix as a temporary workaround until we can update dEQP.

> Fixes several tests in KHR-GLES31.core.constant_expressions and
> KHR-GLES31.core.shader_macros. They now correctly return unsupported.

Bug: angleproject:4143
Change-Id: I4001b30ab609ac4c9be2ce06c2d3c0dca75b64e6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2375965
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>