mingw: remove reference to secret package comparison script
    
    The guidelines reference a script for comparing native and mingw RPMs,
    however, there is no pointer to where it is. This script has existed as
    an attachment in a mailing list thread a great many years ago, but was
    never formalized into something that MinGW maintainers are expected to
    use. Remove the reference to it so maintainers don't go looking for
    something they will struggle to find.
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>