[bbr] remove Backbone Router DUA ND Proxying feature (#13136)

This commit removes all code, configurations, APIs, and tests related
to the OPENTHREAD_CONFIG_BACKBONE_ROUTER_DUA_NDPROXYING_ENABLE feature.

Specifically, the following changes were made:
- Removed DUA ND Proxying Backbone Router configuration option and the
  related OPENTHREAD_CONFIG_NDPROXY_TABLE_ENTRY_NUM definition.
- Removed CLI commands: `bbr mgmt dua` and the proactive backbone
  notification fake command `/b/ba`.
- Removed NdProxyTable and bbr_manager DUA ND Proxying implementation.
- Removed public/internal APIs for ND Proxying and proactive backbone
  notifications.
- Deleted ndproxy_table source files and unit tests.
- Simplified CMake and GN build files to remove deleted targets.

src/core/thread/tmf.cpp

@@ -150,9 +150,6 @@ bool Agent::HandleResource(const char *aUriPath, Msg &aMsg)
 #if OPENTHREAD_CONFIG_BACKBONE_ROUTER_MULTICAST_ROUTING_ENABLE
         Case(kUriMlr, BackboneRouter::Manager);
 #endif
-#if OPENTHREAD_CONFIG_BACKBONE_ROUTER_DUA_NDPROXYING_ENABLE
-        Case(kUriDuaRegistrationRequest, BackboneRouter::Manager);
-#endif
 #endif
 #if OPENTHREAD_CONFIG_BLE_TCAT_ENABLE
         Case(kUriTcatEnable, MeshCoP::TcatAgent);