Do not manually close a pull request

Because of the manual closing, the pull request is occasionally set to
"closed" rather than "merged" even though it was successful. This is
because there is a little latency between the fast-forwarding and the
automatic transition to "merged".

If a bad thing happens, it would be better to leave it "open".

bors.py

@@ -597,13 +597,6 @@ def advance_master_ref_to_test(self, pulls):
             self.add_comment(self.sha, s)
             self.set_error(s)
 
-        try:
-            self.dst().pulls(self.num).patch(state="closed")
-            self.closed = True
-        except github.ApiError:
-            self.log.info("closing failed; auto-closed after merge?")
-            pass
-
         return ret
 
     def try_advance(self, pulls, cfg):