[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.

script/test

@@ -350,7 +350,6 @@ do_build_otbr_docker()
         "-DOT_SRP_CLIENT=ON"
         "-DOT_FULL_LOGS=ON"
         "-DOT_UPTIME=ON"
-        "-DOTBR_DUA_ROUTING=ON"
         "-DOTBR_DHCP6_PD=ON"
     )
     local args=(