Remove an over-zealous DCHECK in jsep_transport.h

This fires in downstream tests when re-negotiation of a datagram
transport completes.

Bug: webrtc:9719
Change-Id: Ie7337e7dc33e41a83da37d3ef2fda596d9107256
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/140821
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28183}

pc/jsep_transport.h

@@ -317,7 +317,6 @@ class JsepTransport : public sigslot::has_slots<>,
     } else if (unencrypted_rtp_transport_) {
       return unencrypted_rtp_transport_.get();
     } else {
-      RTC_DCHECK(media_transport_);
       return nullptr;
     }
   }