[clang] [Clang] Remove workaround for libstdc++4.7 (PR #139693)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> We document libstdc++4.8 as the minimum supported version

It took me quite a while to find that documentation, we probably should make this a bit more prominent from the getting started page at some point: https://github.com/llvm/llvm-project/blob/c14acb74423a577e10bbb635109851742e77444f/clang/docs/Toolchain.rst?plain=1#L343

https://github.com/llvm/llvm-project/pull/139693