fix(conference) remove no longer needed code

There is no need for setting the availability of desktop sharing anymore. It can
now be detected on the spot.

The reson for the previous code was that way back when browser extensions were
needed, it was possible to start a conference without desktopo sharing support
and get it afterwards. This is no longer the case.

react/features/toolbox/components/web/Toolbox.js

@@ -25,6 +25,7 @@ import {
     IconShareDesktop,
     IconShareVideo
 } from '../../../base/icons';
+import JitsiMeetJS from '../../../base/lib-jitsi-meet';
 import {
     getLocalParticipant,
     getParticipants,
@@ -1403,7 +1404,7 @@ class Toolbox extends Component<Props, State> {
  */
 function _mapStateToProps(state) {
     const { conference, locked } = state['features/base/conference'];
-    let { desktopSharingEnabled } = state['features/base/conference'];
+    let desktopSharingEnabled = JitsiMeetJS.isDesktopSharingEnabled();
     const {
         callStatsID,
         enableFeaturesBasedOnToken