Fix rfb._cleanup() (#743)

We don't have to check for _display or context here since this is a
private function which is never called under such circumstances. This
solves problems caused by display.get_context() which was previously
removed in e549ae074fcea9febde32c0fa260a64c15cc1b8e.

core/rfb.js

@@ -444,7 +444,6 @@
         },
 
         _cleanup: function () {
-            if (this._display && this._display.get_context()) {
             if (!this._view_only) { this._keyboard.ungrab(); }
             if (!this._view_only) { this._mouse.ungrab(); }
             this._display.defaultCursor();
@@ -453,7 +452,6 @@
                 // debug mode
                 this._display.clear();
             }
-            }
         },
 
         /*