Make removal of obsolete source packages configurable

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -2733,6 +2733,7 @@ class Britney(object):
         # run the auto hinter
         self.auto_hinter()
 
+        if getattr(self.options, "remove_obsolete", "yes") == "yes":
             # obsolete source packages
             # a package is obsolete if none of the binary packages in testing
             # are built by it
@@ -2747,7 +2748,7 @@ class Britney(object):
             removals = [ MigrationItem("-%s/%s" % (source, sources[source].version))
                          for source in sources if source not in used
                        ]
-        if len(removals) > 0:
+            if removals:
                 self.output_write("Removing obsolete source packages from testing (%d):\n" % (len(removals)))
                 self.do_all(actions=removals)