Reland "remove mslabel and mslabel ssrc-specific attributes"

This reverts commit f0a34f2a3061da6ecb302af122a1126d0a65a1b0.

Reason for revert: Fixed downstream projects to not require those fields.

Original change's description:
> Revert "remove mslabel and mslabel ssrc-specific attributes"
>
> This reverts commit e3f257c4ee2079dee14ec8425eec691db3a9757c.
>
> Reason for revert: Breaks downstream projects
>
> Original change's description:
> > remove mslabel and mslabel ssrc-specific attributes
> >
> > Removes support for parsing and serializing
> >   a=ssrc:1 mslabel:stream
> >   a=ssrc:1 label:track
> > which have been superceeded by
> >   a=ssrc:1 msid:stream track
> > a long time ago.
> >
> > Bug: webrtc:7110
> > Change-Id: I3aca47728098b6e7e049b82ed34c59426d411c41
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168244
> > Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> > Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> > Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#30801}
>
> TBR=kthelgason@webrtc.org,hta@webrtc.org,philipp.hancke@googlemail.com
>
> Change-Id: Ibd0ad11d2dee9f54bacab3dcca61dedccfc2c120
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: webrtc:7110
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170620
> Reviewed-by: Artem Titov <titovartem@webrtc.org>
> Commit-Queue: Artem Titov <titovartem@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#30805}

Change-Id: Iec4bd460b35ce49cdb03dd607d2c8b1dee6210ab
Bug: webrtc:7110
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170622
Reviewed-by: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36640}
diff --git a/sdk/objc/unittests/RTCSessionDescriptionTest.mm b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
index 25d7ffe..cb4a01c 100644
--- a/sdk/objc/unittests/RTCSessionDescriptionTest.mm
+++ b/sdk/objc/unittests/RTCSessionDescriptionTest.mm
@@ -88,8 +88,6 @@
           "a=maxptime:60\r\n"
           "a=ssrc:1504474588 cname:V+FdIC5AJpxLhdYQ\r\n"
           "a=ssrc:1504474588 msid:ARDAMS ARDAMSa0\r\n"
-          "a=ssrc:1504474588 mslabel:ARDAMS\r\n"
-          "a=ssrc:1504474588 label:ARDAMSa0\r\n"
           "m=video 9 UDP/TLS/RTP/SAVPF 100 116 117 96\r\n"
           "c=IN IP4 0.0.0.0\r\n"
           "a=rtcp:9 IN IP4 0.0.0.0\r\n"
@@ -118,12 +116,8 @@
           "a=ssrc-group:FID 498297514 1644357692\r\n"
           "a=ssrc:498297514 cname:V+FdIC5AJpxLhdYQ\r\n"
           "a=ssrc:498297514 msid:ARDAMS ARDAMSv0\r\n"
-          "a=ssrc:498297514 mslabel:ARDAMS\r\n"
-          "a=ssrc:498297514 label:ARDAMSv0\r\n"
           "a=ssrc:1644357692 cname:V+FdIC5AJpxLhdYQ\r\n"
-          "a=ssrc:1644357692 msid:ARDAMS ARDAMSv0\r\n"
-          "a=ssrc:1644357692 mslabel:ARDAMS\r\n"
-          "a=ssrc:1644357692 label:ARDAMSv0\r\n";
+          "a=ssrc:1644357692 msid:ARDAMS ARDAMSv0\r\n";
 }
 
 @end