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.

closed_conn_test.go

@@ -14,8 +14,6 @@ var _ = Describe("Closed local connection", func() {
 	It("tells its perspective", func() {
 		conn := newClosedLocalConn(nil, protocol.PerspectiveClient, utils.DefaultLogger)
 		Expect(conn.getPerspective()).To(Equal(protocol.PerspectiveClient))
-		// stop the connection
-		conn.shutdown()
 	})
 
 	It("repeats the packet containing the CONNECTION_CLOSE frame", func() {