umqtt.robust: Remove reference to missing example.

It looks like this example file was not added to the original commit back
in 6190cec14a.

Fixes issue #320.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

micropython/umqtt.robust/example_sub_robust.py

@@ -19,8 +19,7 @@ c.set_callback(sub_cb)
 #
 # There can be a problem when a session for a given client exists,
 # but doesn't have subscriptions a particular application expects.
-# In this case, a session needs to be cleaned first. See
-# example_reset_session.py for an obvious way how to do that.
+# In this case, a session needs to be cleaned first.
 #
 # In an actual application, it's up to its developer how to
 # manage these issues. One extreme is to have external "provisioning"