Ubuntu: remove obsolete dependency

It appears that this dependency is no longer provided (as of Mantis), but also
no longer needed (as a build without it appears to completed). Let's see if
that fixes our Mantis build issue.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

packaging/ubuntu/debian/control

@@ -4,7 +4,6 @@ Priority: optional
 Maintainer: Subsurface CI Bot <dirk@subsurface-divelog.org>
 Build-Depends: asciidoc,
                debhelper (>= 9),
-               libgconf2-dev,
                libtool,
                libxml2-dev,
                libxslt-dev,