Remove unused _ssh_to_server from test_stamp_pattern

Commit abd154c87a75bd4100bfe72c2ed3f5d31d5cf0a2 removed the private
methods that were using _ssh_to_server so we can also remove that here
since it's now unused.

Change-Id: I31c849a2a9153fe5b93c54563b485150bc06f9c5
diff --git a/tempest/scenario/test_stamp_pattern.py b/tempest/scenario/test_stamp_pattern.py
index b9d5fcc..a4f9896 100644
--- a/tempest/scenario/test_stamp_pattern.py
+++ b/tempest/scenario/test_stamp_pattern.py
@@ -74,9 +74,6 @@
     def _add_keypair(self):
         self.keypair = self.create_keypair()
 
-    def _ssh_to_server(self, server_or_ip):
-        return self.get_remote_client(server_or_ip)
-
     def _create_volume_snapshot(self, volume):
         snapshot_name = data_utils.rand_name('scenario-snapshot')
         _, snapshot = self.snapshots_client.create_snapshot(