libdmx: remove obsolete library

libdmx: remove obsolete library

The Xdmx server was removed from the xorg-server sources in xorg-server
21 (released Oct. 2021), so this is only useful for communicating with
Xdmx from the 1.20 and older releases.

Also remove the DEPENDS on libdmx in xdpyinfo for the same reason.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>