/etc/default/xen: Handle with ucf

/etc/default/xen: Handle with ucf

We reintroduced this file in
  "d/xen-utils-common.install: ship /etc/default/xen"
  eg 2f34db35dd27abb4280d38ebc4464c21f64df8c9

But I had forgotten that this file was previously shipped and handled
by ucf; and we have machinery to try to remove it.

This leaves the following possible cases:
(a) stretch: file handled by ucf
(b) older buster: file not shipped, ucf postinst action not done
   | file remains recorded by ucf and ucfr, but that the original
   | version of the file is no longer present on the user's machine
(c) newer buster: file shipped, file recorded by ucf with one
   old version and as dpkg conffile by newer version

(a) and (b) will be handled correctly by just using ucf in the normal
way.

(c) xxx needs testing

So:
 * Drop the special call to ucf-remove-fixup; retain the call to ucf
 * Switch to shipping the file in /usr/share as expected by ucf

We do not remove the file's entry from not-installed - see the
comment relating to #831786.

Signed-off-by: Ian Jackson <ian.jackson@citrix.com>