Remove unused method checkoutLocalBranch

The method checkoutLocalBranch is unused and also has a todo for
retirement of this method. It looks like this retirement was done
without removing this method.

Change-Id: Id30a927f583951cddd928da02898c1316972d72a
diff --git a/zuul/merger/merger.py b/zuul/merger/merger.py
index 07f3e69..5e102b4 100644
--- a/zuul/merger/merger.py
+++ b/zuul/merger/merger.py
@@ -261,14 +261,6 @@
         repo.git.checkout(ref)
         return repo.head.commit
 
-    def checkoutLocalBranch(self, branch):
-        # TODO(jeblair): retire in favor of checkout
-        repo = self.createRepoObject()
-        # Perform a hard reset before checking out so that we clean up
-        # anything that might be left over from a merge.
-        reset_repo_to_head(repo)
-        repo.heads[branch].checkout()
-
     def cherryPick(self, ref):
         repo = self.createRepoObject()
         self.log.debug("Cherry-picking %s" % ref)