Remove pointless Display.clear()

It served no meaningful purpose and it had bugs. So let's remove it
rather than try to fix it.

kasmweb/core/rfb.js

@@ -193,7 +193,6 @@ export default class RFB extends EventTargetMixin {
             throw exc;
         }
         this._display.onflush = this._onFlush.bind(this);
-        this._display.clear();
 
         this._keyboard = new Keyboard(this._canvas);
         this._keyboard.onkeyevent = this._handleKeyEvent.bind(this);