fix undef of QT_NO_CAST_FROM_ASCII and QT_NO_CAST_TO_ASCII

the original -U... does not work with MSVC 2010, nor seems the appropriate
/U syntax (maybe I missed something there though)
anyway, using the remove_definitions cmake macro seems to work - maybe
this is the most generic way and should be preferred in general? (I did
not remove the old -U definitions since I don't know where they may be
needed instead)