Revert "introduce an unsupported content description type"

This reverts commit 239f92ecf7fc8ca27e0376dd192b33ce33377b3c.

Reason for revert: Breaks downstream projects.

Original change's description:
> introduce an unsupported content description type
>
> This carries around unsupported content descriptions
> (i.e. things where webrtc does not understand the media type
> or protocol) in a special data type so that a rejected content or
> mediasection is added to the answer SDP.
>
> BUG=webrtc:3513
>
> Change-Id: Ifc4168eae11e899f2504649de5e1eecb6801a9fb
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/179082
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
> Cr-Commit-Position: refs/heads/master@{#32410}

TBR=kthelgason@webrtc.org,hta@webrtc.org,philipp.hancke@googlemail.com

Change-Id: I055fe001fe2757d79be7c304eccc43a8e3104f69
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:3513
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/188581
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32411}

pc/rtc_stats_collector_unittest.cc

@@ -2597,7 +2597,6 @@ class RTCStatsCollectorTestWithParamKind
       case cricket::MEDIA_TYPE_VIDEO:
         return "Video";
       case cricket::MEDIA_TYPE_DATA:
-      case cricket::MEDIA_TYPE_UNSUPPORTED:
         RTC_NOTREACHED();
         return "";
     }
@@ -2656,7 +2655,6 @@ class RTCStatsCollectorTestWithParamKind
         return;
       }
       case cricket::MEDIA_TYPE_DATA:
-      case cricket::MEDIA_TYPE_UNSUPPORTED:
         RTC_NOTREACHED();
     }
   }