Always reject packages with unsatisfiable dependencies

Partially revert commit ac66e311 which caused packages with unsatisfiable
dependencies to only get rejected if they were not in testing. In Ubuntu we
always want to block those.

britney.py

@@ -1574,7 +1574,7 @@ class Britney(object):
                 # find unsatisfied dependencies for the binary package
                 if binary_u.architecture != 'all' or arch in self.options.nobreakall_arches:
                     is_valid = self.excuse_unsat_deps(pkg, src, arch, suite, excuse)
-                    if not is_valid and not source_t:
+                    if not is_valid:
                         excuse.is_valid = False
 
             # if there are out-of-date packages, warn about them in the excuse and set is_valid