Don't update downstream caps in upstream RPL_WELCOME handler

Prior to being registered, upstreamConn.handleMessage doesn't run
in the user goroutine, it runs in a goroutine specific to the
network. Thus we shouldn't access any user data structure from
there.

downstreamConn.updateSupportedCaps is already called from the
eventUpstreamConnected handler in user.run, the call being removed
was unnecessary.

Closes: https://todo.sr.ht/~emersion/soju/108

upstream.go

@@ -590,10 +590,6 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 		uc.registered = true
 		uc.logger.Printf("connection registered")
 
-		uc.forEachDownstream(func(dc *downstreamConn) {
-			dc.updateSupportedCaps()
-		})
-
 		if len(uc.network.channels) > 0 {
 			var channels, keys []string
 			for _, ch := range uc.network.channels {