Discovery shouldn't tag relays

[vyzo]

We have this code: https://github.com/libp2p/go-libp2p-circuit/blob/master/notify.go#L49
which tags all relays discovered with passive discovery.

This was useful before autorelay, but it seems to result in a large number of idle connections.

What was the rationale for it?
I suggest we remove this tag.

cc @whyrusleeping who added the tag, @Stebalien

[Stebalien]

We probably added it assuming that we're more likely to connect to peers behind relays than not. These nodes are strictly more useful.

Given the low tag value, I'm happy either way.

[vyzo]

yeah, not binding. I am not dead set on removing them either.