Refactor: based, libcib: Drop F_CIB_CALLBACK_TOKEN

This isn't used for anything except to assign client->userdata to a
pcmk__client_t object in based_remote:cib_handle_remote_msg(). The
client object is internal to pacemaker-based, and nothing there actually
used client->userdata after it was assigned.

Whatever this was originally used for seems to have been superseded by
F_CIB_CLIENTID.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>