isisd: fix display of "isis bfd" in the running configuration

isisd: fix display of "isis bfd" in the running configuration

Since this command is modeled using YANG, it's already displayed
as part of the call to nb_cli_show_dnode_cmds(). Calling the
'isis_circuit_config_write' hook was only making that command
to be displayed twice.

The aforementioned hook is still necessary for fabricd, which wasn't
converted to the new northbound model yet.

Fixes #6281.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>