Don't manually close PRs after merging

Pair programmed with @Vagabond.

We tested this and github will automatically set the status of the
pull-request to closed/merged if the HEAD of the feature branch is
contained in the target branch. This happens when we fast-forward the
target branch to the merge-sha. And specifically, it looks at the target
branch, not whatever the default branch is set to in github
(master/develop).

bors.py

@@ -516,11 +516,6 @@ def advance_target_ref_to_test(self):
             self.dst().git().refs().heads(self.target_ref).patch(sha=self.merge_sha,
                                                                  force=False)
             self.add_comment(self.sha, 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?")
             try:
                 self.dst().git().refs().heads(self.test_ref).delete()
             except github.ApiError: