[autotest] Simplify access to readonly_connection

After refactoring the way readonly_connection works in
CL:223393 some uses of it can be shortened.

BUG=None
DEPLOY=apache,scheduler,host_scheduler,shard_client
TEST=Ran suites, syncdb, apache restart and dummy suite.

Change-Id: I461345ecb362bd54740417f0016a64072581a513
Reviewed-on: https://chromium-review.googlesource.com/223394
Tested-by: Jakob Jülich <jakobjuelich@chromium.org>
Reviewed-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Jakob Jülich <jakobjuelich@chromium.org>
diff --git a/frontend/tko/models.py b/frontend/tko/models.py
index af88dee..81e2928 100644
--- a/frontend/tko/models.py
+++ b/frontend/tko/models.py
@@ -65,7 +65,7 @@
 
         """
         sql, params = self._get_group_query_sql(query, group_by)
-        cursor = readonly_connection.connection().cursor()
+        cursor = readonly_connection.cursor()
         cursor.execute(sql, params)
         field_names = self._get_column_names(cursor)
         row_dicts = [dict(zip(field_names, row)) for row in cursor.fetchall()]
@@ -116,7 +116,7 @@
 
         """
         sql, params = self._get_num_groups_sql(query, group_by)
-        cursor = readonly_connection.connection().cursor()
+        cursor = readonly_connection.cursor()
         cursor.execute(sql, params)
         return self._cursor_rowcount(cursor)