xenomai: use install-user to remove patch

As noted by "George" in bug #6416, the xenomai-do-not-install-devices
patch is not needed, since Xenomai provides a "install-user" target to
only install the libraries/headers and not the device files. This
patch changes xenomai.mk to use 'install-user' and removes the useless
patch.

Fixes bug #6416.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>