[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.

.github/workflows/otci.yml

@@ -80,7 +80,7 @@ jobs:
         PYTHONPATH=./tests/scripts/thread-cert pytype tools/otci
     - name: Build
       run: |
-        ./script/cmake-build simulation -DOT_THREAD_VERSION=1.4 -DOT_DUA=ON -DOT_MLR=ON -DOT_BACKBONE_ROUTER=ON \
+        ./script/cmake-build simulation -DOT_THREAD_VERSION=1.4 -DOT_MLR=ON -DOT_BACKBONE_ROUTER=ON \
         -DOT_CSL_RECEIVER=ON -DOT_SIMULATION_VIRTUAL_TIME=${VIRTUAL_TIME}
     - name: Install OTCI Python Library
       run: |