gms/gossiper: Cleanup notify_failure_detector()
gms/gossiper: Cleanup notify_failure_detector()

Now that we have get_endpoint_state_for_endpoint_ptr(), which does not
return a copy and allows mutating the actual state, we can use it
instead of repeating the lookup code.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla