Revert "[build] Suppress new -Wunnecessary-virtual-specifier warning"

This reverts commit 0cf871bc4134fd8bd4dc600e0780207f30ccdfef.

Reason for revert:
It was turned off by default again in
https://github.com/llvm/llvm-project/pull/134105
also the chromeos tip-of-tree bots are unhappy about the flag:
e.g. https://ci.chromium.org/ui/p/chromium/builders/ci/ToTChromeOS%20(dbg)/15986/overview

Original change's description:
> [build] Suppress new -Wunnecessary-virtual-specifier warning
>
> Bug: 407622661
> Change-Id: I27948beb53cd8c2206d6371b35511d295ee2ed5b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6421738
> Auto-Submit: Hans Wennborg <hans@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Devon Loehr <dloehr@google.com>
> Cr-Commit-Position: refs/heads/main@{#1440917}

Bug: 407622661
Change-Id: Ib18c822e98f4ac8250a6be46e1fe2ef80847b3e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6439161
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Devon Loehr <dloehr@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1444137}