staging: MAINTAINERS: remove obsolete IPX staging directory

The IPX code was removed from staging back in November 2017, but the
MAINTAINERS entry stuck around.  Remove the invalid directory from the
file as it does not actually point to anything anymore.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>