git_config: fix _SaveJson typo

Change-Id: I35ca2b3733e6d1508669f9a6690c6645c582912e

git_config.py

@@ -280,7 +280,7 @@ class GitConfig(object):
       finally:
         fd.close()
     except (IOError, TypeError):
-      if os.path.exists(self.json):
+      if os.path.exists(self._json):
         os.remove(self._json)
 
   def _ReadGit(self):