mgr/cephadm: don't include agents in CEPHADM_FAILED_DAEMON

mgr/cephadm: don't include agents in CEPHADM_FAILED_DAEMON

They already have their own, more strict health warning.
There's very few cases they would show up in failed daemon
health check but not agent down health check and even if
they did it would be temporary. Also, agents marked as down
will automatically (before this change) be marked as failed
even if they don't meet the typical criteria for failed
(systemd status is in error)

Fixes: https://tracker.ceph.com/issues/53448
Signed-off-by: Adam King <adking@redhat.com>