Potential cleanup re REG_LEGAL_CHANGE_FILTER

In https://github.com/msys2-contrib/cpython-mingw/commit/15a5dad86e97f4195c38f9bdb3645662e6e8a865

REG_LEGAL_CHANGE_FILTER should no longer be needed with recent mingw-w64.
SIZEOF_HKEY also should in theory be set by pyconfig.h, so maybe could also be removed.