[dua] completely remove DUA features and configurations (#13191)

This commit removes the OPENTHREAD_CONFIG_TMF_PROXY_DUA_ENABLE feature
and all associated code, tests, CLI commands, and harness references.

Changes:
- Removed OPENTHREAD_CONFIG_TMF_PROXY_DUA_ENABLE definition and all
  assert/preprocessor checks.
- Completely deleted dua_manager.cpp and dua_manager.hpp.
- Removed DUA registration notifying and request URI paths.
- Cleaned up all references to Domain Unicast Address (DUA) across
  child management, notifier, time ticker, and MLE.
- Removed DUA commands and logic from the CLI and Python cert tests
  (including packet verifier).
- Verified that the entire codebase compiles clean and all tests
  successfully pass using the Nexus test suite.

tests/scripts/thread-cert/config.py

@@ -94,7 +94,6 @@ class ADDRESS_TYPE(Enum):
     RLOC = 'RLOC'
     ALOC = 'ALOC'
     ML_EID = 'ML_EID'
-    DUA = 'DUA'
     BACKBONE_GUA = 'BACKBONE_GUA'
     BACKBONE_LINK_LOCAL = 'BACKBONE_LINK_LOCAL'
     OMR = 'OMR'
@@ -139,7 +138,6 @@ ADDRESS_QUERY_INITIAL_RETRY_DELAY = 15
 DEFAULT_CHILD_TIMEOUT = 6
 VIRTUAL_TIME = int(os.getenv('VIRTUAL_TIME', 0))
 PARENT_AGGREGATIOIN_DELAY = 5
-DUA_DAD_DELAY = 5
 DEFAULT_BBR_REGISTRATION_JITTER = 2
 DEFAULT_ROUTER_SELECTION_JITTER = 1