remove shutdown method on the Connection (#4249)

There's no need to have a dedicated shutdown method, as the use case
(shutting down an outgoing connection attempt on context cancellation)
can be achieved by using Connection.destroy.

client.go

@@ -232,7 +232,7 @@ func (c *client) dial(ctx context.Context) error {
 
 	select {
 	case <-ctx.Done():
-		c.conn.shutdown()
+		c.conn.destroy(nil)
 		return context.Cause(ctx)
 	case err := <-errorChan:
 		return err