[clang] [llvm] Revert "Enable unnecessary-virtual-specifier by default" (PR #134105)

[Nikolas Klauser via llvm-commits]

philnik777 wrote:

I think we need some better transition story here. Maybe don't enable the warning by default for time until we've had time to update the libc++ containers?

https://github.com/llvm/llvm-project/pull/134105