ddb: Don't flag breakpoint/watchpoint commands as MEMSAFE

They could potentially be abused to overwrite kernel memory, so
shouldn't be accessible when mac_ddb is loaded.

Reviewed by: mhorne
Fixes: bc4ea61d55cb ("ddb: tag core commands with DB_CMD_MEMSAFE")
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37105