[tests] remove obsolete DUA-dependent backbone tests (openthread#13141)

This commit removes the obsolete Backbone Router (BBR) certification
tests:
- tests/scripts/thread-cert/backbone/bbr_5_11_01.py
- tests/scripts/thread-cert/backbone/
  test_mlr_multicast_routing_across_thread_pans.py

These tests are removed because DUA (Domain Unicast Address) routing
features (specifically DUA ND Proxying) have been deprecated and
removed from the codebase. Since these features are no longer
supported, the corresponding certification and validation tests are
no longer valid or runnable.

Author: jwhui