debian: Remove duplicate clean command

`dh clean` invokes `dh_testdir`, `dh_auto_clean` and `dh_clean`. We
don't need to invoke dh_clean twice.
Acked-by: Travis Cross <tc@traviscross.com>

debian/rules

@@ -55,7 +55,6 @@ clean:
 	dh $@
 
 override_dh_auto_clean:
-	dh_clean
 
 .stamp-bootstrap:
 	@$(call show_vars)