[tests] remove border router MATN tests (openthread#13306)

Remove MATN border router certification tests as they have been
replaced by equivalent tests in the ot-br-posix repository under:
https://github.com/openthread/ot-br-posix/tree/main/tests/scripts/expect

The new tests run using the docker-in-docker framework.

Deleted tests:
- MATN_02_MLRFirstUse.py
- MATN_03_InvalidCommissionerDeregistration.py
- MATN_04_MulticastListenerTimeout.py
- MATN_05_ReregistrationToSameMulticastGroup.py
- MATN_09_DefaultBRMulticastForwarding.py
- MATN_12_HopLimitProcessing.py
- MATN_15_ChangeOfPrimaryBBRTriggersRegistration.py
- MATN_16_LargeNumberOfMulticastGroupSubscriptionsToBBR.py

Author: jwhui

.github/workflows/otbr.yml

@@ -63,12 +63,6 @@ jobs:
             packet_verification: 2
             nat64: 0
             description: ""
-          - otbr_mdns: "mDNSResponder"
-            otbr_trel: 0
-            cert_scripts: ./tests/scripts/thread-cert/border_router/MATN/*.py
-            packet_verification: 1
-            nat64: 0
-            description: "MATN"
           - otbr_mdns: "mDNSResponder"
             otbr_trel: 0
             cert_scripts: ./tests/scripts/thread-cert/border_router/internet/*.py