Modify Bundle logic to not add & destroy extra transport at add-track

If a bundle is established, then per JSEP, the offerer is required to
include the new track in the bundle, and per BUNDLE, the answerer has
to either accept the track as part of the bundle or reject the track;
it cannot move it out of the group. So we will never need the transport.

Bug: webrtc:12837
Change-Id: I41cae74605fecf454900a958776b95607ccf3745
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/221601
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34290}

pc/jsep_transport_controller_unittest.cc

@@ -2051,7 +2051,6 @@ TEST_F(JsepTransportControllerTest, ChangeTaggedMediaSectionMaxBundle) {
   EXPECT_TRUE(transport_controller_
                   ->SetLocalDescription(SdpType::kOffer, local_reoffer.get())
                   .ok());
-
   std::unique_ptr<cricket::SessionDescription> remote_reanswer(
       local_reoffer->Clone());
   EXPECT_TRUE(