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

[Tobias Hieta via llvm-commits]

tru 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/pull/145683