x11/libxcvt: avoid conflict with xorg-server

Nowadays, kde5 depends on both xorg-server and xwayland-devel.
Once upstream of both switches to libxcvt and xwayland-devel is
updated it'd make kde5 uninstallable. Backporting to xorg-server
is complicated by numerious patch conflicts and autotools.

Stop installing cvt(1) as Xwayland doesn't use xorg.conf(5).