[bbr] remove domain prefix support from stack and harness (#13203)

This commit removes all Domain Prefix configuration and management logic
from the OpenThread stack, CLI commands, unit tests, and GRL harness
THCI wrapper.

- Removed public Backbone Router Domain Prefix APIs.
- Removed Domain Prefix flag ('mDp') and 'D' flag parser/formatter
  from core network data types, Spinel, and CLI.
- Cleaned up local Backbone Router and Leader logic to exclude Domain
  Prefix configuration, tracking, and events.
- Updated RoutingManager prefix advertisement (RIO) to exclude
  special handling for Domain Prefix.
- Updated CLI documentation to remove Domain Prefix references.
- Removed domain prefix helper methods from python test certification
  scripts.
- Removed auto-addition of default domain prefix and D flag support
  from GRL harness OpenThread.py.

tests/scripts/thread-cert/node.py

@@ -1608,14 +1608,6 @@ class NodeImpl:
         self.send_command(cmd)
         self._expect_done()
 
-    def set_domain_prefix(self, prefix, flags='prosD'):
-        self.add_prefix(prefix, flags)
-        self.register_netdata()
-
-    def remove_domain_prefix(self, prefix):
-        self.remove_prefix(prefix)
-        self.register_netdata()
-
     def multicast_listener_list(self) -> Dict[IPv6Address, int]:
         cmd = 'bbr mgmt mlr listener'
         self.send_command(cmd)