git cl: remove more unused func kwargs.

R=ehmaldonado

Change-Id: Ia35da60783cfbfd472646176d8fc669d8f4d9839
Reviewed-on: https://chromium-review.googlesource.com/1185989
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -2704,8 +2704,8 @@ class _GerritChangelistImpl(_ChangelistCodereviewBase):
     self._detail_cache.setdefault(issue, []).append((frozenset(options), data))
     return data
 
-  def _GetChangeCommit(self, issue=None):
-    issue = issue or self.GetIssue()
+  def _GetChangeCommit(self):
+    issue = self.GetIssue()
     assert issue, 'issue is required to query Gerrit'
     try:
       data = gerrit_util.GetChangeCommit(self._GetGerritHost(), str(issue))