d/control: Remove outdated debconf version constraint

debconf 1.5.34 is older than Debian squeeze, thus no longer needed.
Also, debconf is not needed as Pre-Depends, as it is not called from
`preinst`.

An appropriate dependency will be added by `dh_installdebconf`.

debian/control

@@ -35,8 +35,7 @@ Description: BGP swiss army knife of networking - Python 3 module
 
 Package: exabgp
 Architecture: all
-Pre-Depends: debconf (>= 1.5.34),
-             dpkg (>= 1.15.7.2),
+Pre-Depends: dpkg (>= 1.15.7.2),
              ${misc:Pre-Depends}
 Depends: adduser,
          python3-exabgp (= ${binary:Version}),