Remove broken isScreenSharing function. (#2057)

modules/RTC/JitsiTrack.js

         return Promise.resolve();
     }
 
-    /**
-     * Returns true if this is a video track and the source of the video is a
-     * screen capture as opposed to a camera.
-     */
-    isScreenSharing() {
-        // FIXME: Should be fixed or removed.
-    }
-
     /**
      * Returns id of the track.
      * @returns {string|null} id of the track or null if this is fake track.

types/auto/modules/RTC/JitsiTrack.d.ts

      * @returns {Promise}
      */
     dispose(): Promise<any>;
-    /**
-     * Returns true if this is a video track and the source of the video is a
-     * screen capture as opposed to a camera.
-     */
-    isScreenSharing(): void;
     /**
      * Returns id of the track.
      * @returns {string|null} id of the track or null if this is fake track.