This reverts commit bab2b3de.

Reason for revert:
A fixed clang version has been rolled in, so this flag should no longer
be necessary.

Original change's description:
> Vulkan: Disable global merge for volk
>
> This optimization pass breaks the build by exporting volk's internal
> binding symbols from libGLESv2.so. Since the test binary also links
volk,
> this causes a SEGV as the wrong (uninitialized) function pointer is
> called.
>
> This is an LLVM bug and will be fixed upstream:
> https://reviews.llvm.org/D73235
>
> Bug: angleproject:4351
> Change-Id: Iebe0eabd975f8109231df30eef8c9074af8190e9
> Reviewed-on:
https://chromium-review.googlesource.com/c/angle/angle/+/2023910
> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
> Commit-Queue: Michael Spang <spang@chromium.org>

TBR=spang@chromium.org,syoussefi@chromium.org,jmadill@chromium.org

Bug: angleproject:4351
Change-Id: Ie832d7d42fab7c8360f2d4d548f1708164c60d3f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2076138
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>