Revert "Fix unsynchronized access to mid_to_transport_ in JsepTransportController"

This reverts commit 6cd405850467683cf10d05028ea0f644a68a91a4.

Reason for revert: Breaks WebRTC Chromium FYI Bots

First failure:
https://ci.chromium.org/p/chromium/builders/webrtc.fyi/WebRTC%20Chromium%20FYI%20Android%20Tests%20%28dbg%29%20%28L%20Nexus5%29/1925

Failed tests:
WebRtcDataBrowserTest.CallWithSctpDataAndMedia
WebRtcDataBrowserTest.CallWithSctpDataOnly

Original change's description:
> Fix unsynchronized access to mid_to_transport_ in JsepTransportController
>
> * Added several thread checks to JTC to help with programmer errors.
> * Avoid a few Invokes() to the network thread here and there such
>   as for fetching sctp transport name for getStats(). The transport
>   name is now cached when it changes on the network thread.
> * JsepTransportController instances now get deleted on the network
>   thread rather than on the signaling thread + issuing an Invoke()
>   in the dtor.
> * Moved some thread hops from JTC over to PC which is where the problem
>   exists and also (imho) makes it easier to see where hops happen in
>   the PC code.
> * The sctp transport is now started asynchronously when we push down the
>   media description.
> * PeerConnection proxy calls GetSctpTransport directly on the network
>   thread instead of to the signaling thread + blocking on the network
>   thread.
> * The above changes simplified things for webrtc::SctpTransport which
>   allowed for removing locking from that class and delete some code.
>
> Bug: webrtc:9987, webrtc:12445
> Change-Id: Ic89a9426e314e1b93c81751d4f732f05fa448fbc
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/205620
> Commit-Queue: Tommi <tommi@webrtc.org>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#33191}

TBR=tommi@webrtc.org,hta@webrtc.org

Change-Id: I7b2913d5133807589461105cf07eff3e9bb7157e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9987
Bug: webrtc:12445
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/206466
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Commit-Queue: Guido Urdaneta <guidou@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33204}

pc/jsep_transport_controller_unittest.cc

@@ -904,9 +904,6 @@ TEST_F(JsepTransportControllerTest, IceSignalingOccursOnSignalingThread) {
   EXPECT_EQ(2, candidates_signal_count_);
 
   EXPECT_TRUE(!signaled_on_non_signaling_thread_);
-
-  network_thread_->Invoke<void>(RTC_FROM_HERE,
-                                [&] { transport_controller_.reset(); });
 }
 
 // Test that if the TransportController was created with the