remove redundant self-dialing check, simplify starting of dialWorkerLoop

[marten-seemann]

We already check for self dials in

go-libp2p-swarm/swarm_dial.go

Lines 248 to 250 in a32e993

	if p == s.local {
	return nil, ErrDialToSelf
	}

. We don't need to check again before starting the dialWorkerLoop. This allows us to get rid of the error return value.

Also, API-wise, it's always nice when the caller starts the go routine.