Purge a number of Mozilla-specific WebIDL

Did a bunch of grepping for `moz*` and searched for "moz" in rustdoc,
deleting anything that looked mozilla-specific. Now there's nothing left
with the "moz" prefix in rustdoc!

crates/web-sys/webidls/enabled/MediaSource.webidl

@@ -42,6 +42,4 @@ interface MediaSource : EventTarget {
   [Throws]
   void clearLiveSeekableRange();
   static boolean isTypeSupported(DOMString type);
-  [ChromeOnly]
-  readonly attribute DOMString mozDebugReaderData;
 };