Bug 1965779 - Remove GTK iconSize / iconState from icon URI. r=stransky,settings-reviewers

Icon state is now unused (and wasn't used anyhow). Icon sizes are
hardcoded in GTK as per: https://docs.gtk.org/gtk3/enum.IconSize.html

So we can migrate the few usages left to the actual sizes, and remove
the supporting code.

Differential Revision: https://phabricator.services.mozilla.com/D248866

ipc/glue/URIParams.ipdlh

@@ -62,8 +62,6 @@ struct IconURIParams
   nsCString contentType;
   nsCString fileName;
   nsCString stockIcon;
-  int32_t iconSize;
-  int32_t iconState;
   uint32_t iconScale;
   bool? iconDark;
 };