rtcsessiondescription: attributes are not mutable

removes text which claims the attributes were mutable for legacy reasons.

Fixes w3c#1242

Author: fippo

webrtc.html

@@ -3602,8 +3602,7 @@ <h4><dfn>RTCSdpType</dfn></h4>
         <h4><dfn>RTCSessionDescription</dfn> Class</h4>
         <p>The <code>RTCSessionDescription</code> class is used by
         <code><a>RTCPeerConnection</a></code> to expose local and remote
-        session descriptions. Attributes on this interface are mutable for
-        legacy reasons.</p>
+        session descriptions.</p>
         <div>
           <pre class="idl">
           [ Constructor (RTCSessionDescriptionInit descriptionInitDict)]