Revert "peer: use GatheringCompletePromise from Pion"

This reverts commit 9d20fb3.

Author: daniel-abramov

pkg/peer/peer.go

@@ -180,16 +180,12 @@ func (p *Peer[ID]) ProcessSDPOffer(sdpOffer string) (*webrtc.SessionDescription,
 		return nil, ErrCantCreateAnswer
 	}
 
-	// TODO: Do we really need to call `webrtc.GatheringCompletePromise` here?
-	gatherComplete := webrtc.GatheringCompletePromise(p.peerConnection)
-
 	if err := p.peerConnection.SetLocalDescription(answer); err != nil {
 		p.logger.WithError(err).Error("failed to set local description")
 		return nil, ErrCantSetLocalDescription
 	}
 
-	// Block until ICE Gathering is complete.
-	<-gatherComplete
+	// TODO: Do we really need to call `webrtc.GatheringCompletePromise` here?
 
 	sdpAnswer := p.peerConnection.LocalDescription()
 	if sdpAnswer == nil {