simplify QKeyMapper::clearMappings() and do not use the cached locale codec

simplify QKeyMapper::clearMappings() and do not use the cached locale codec

since it is known what locale the X11 input method uses it is redundant to
verify that via XLocaleOfIM() (unless X11 fails to detect what locale is
set for some strange reason).

also not using the cached codec returned by QTextCodec::codecForLocale()
because the locale may change at any time (either environment variable
change or just plain setlocale() call as was done by KCatalog as gettext
workaround) and QKeyMapper has to reflect such changes

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>