[Support] Remove workarounds for building with mingw.org toolchains (#145683)
Assume that mingw builds are made with mingw-w64 headers.
The old mingw.org distribution isn't even accessible at the moment, and
their project hosting site (osdn.net) seems to have been down for a
couple of years, and their old project hosting (at sourceforge.net)
hasn't been updated since 2018.