Drop DMX DDX

It turns out xdmx currently crashes when any client attempts to use GL
and it has been in such state for about 14 years. There was a patch to
fix the problem [1] 4 years ago, but it never got merged. The last
activity on any bugs referring to xdmx has been more than 4 years ago.

Given such situation, I find it unlikely that anyone is still using xdmx
and just having the code is a drain of resources.

[1]: https://lists.x.org/archives/xorg-devel/2017-June/053919.html

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
5 jobs for drop-dmx-ddx in 7 minutes and 1 second (queued for 12 seconds)
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passed debian-buster #13431468

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  Build And Test
passed autotools #13431469

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passed meson #13431470

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passed meson-noglamor #13431471

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passed mingw-cross-build #13431472

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