LP#2051370: remove make_release option to build XUL client

LP#2051370: remove make_release option to build XUL client

This patch removes the -x option in make_release to build
the XUL client, which has ceased to be, is an ex-client. It
doesn't remove the dependencies and subordinate Makefile
that would permit manually building the XUL client; that will
happen later.

Release-note: [Developer] Remove make_release -x option to build XUL client

To test
-------
[1] Install the patch and verify that make_release does not
    advertise an option to build the XUL client.
[2] Run a build and verify that it works.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Jane Sandberg <js7389@princeton.edu>