[llvm] [Support] Remove workarounds for building with mingw.org toolchains (PR #145683)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> > Assume that mingw builds are made with mingw-w64 headers.
> 
> Is this documented in the requirements? It's fine in any case, but I feel like we should have it documented if people come searching for it.

https://github.com/llvm/llvm-project/blob/llvmorg-21-init/llvm/docs/GettingStarted.rst?plain=1#L399-L401

(That doesn't say exactly "mingw-w64" but, close enough to make it clear that it isn't mingw.org.)

https://github.com/llvm/llvm-project/pull/145683