[Support] Remove an outdated MinGW workaround (#145294)

mingw-w64 has had the _HEAPOK define since the initial commits in 2007;
unclear when/where it was added for mingw.org headers, but it does seem
to exist there as well (at least in versions from 2011).

This workaround stems from 53fbecce6e8b7d1f024e3dc6df4160fe9a577ff1 from
2004 - it is no longer relevant today.

GitOrigin-RevId: 7a4b3925598c07f5b4523a701ba6c34e483078d6
1 file changed