sfu: on mobile, hide unsharebutton as 'stop' button can be used in video

To reduce button size on header bar when mobile layout is activated, we hide unsharebutton. User can stop video sharing using video control buttons.

static/sfu.css

@@ -1153,5 +1153,9 @@ header .collapse {
         margin-top: 10px;
     }
 
+    #unsharebutton {
+        display: none;
+    }
+
 }