The `MINGW` variable indicates that the compiler targets MinGW, a GNU
ABI on Windows.  Since commit aff31479 (Modernize GNU compiler info on
Windows, 2009-12-02, v2.8.2~636), we load the `Platform/Windows-GNU`
module for compilers targetin MinGW, so set the variable there instead.
This is equivalent to `Platform/Windows-MSVC` setting the `MSVC`
variable.  Also remove `if(MINGW)` checks from the module, which have
not been necessary since the enclosed logic was moved to that module.

The undocumented `CMAKE_COMPILER_IS_MINGW` internal variable is now
unused, so remove it too.

Fixes: #22647