Remove unused properties and variables

The code that used these were removed in the following commits:

* 9ff86fb718477515ede2d6457f06643935d76bcd (RFB._mouse_arr)
* bb6965f2e60c2301dd638383bdc792d1f10af942 (old_requestAnimationFrame)
* 490d471c53b1791398d6c30f0efce54925939f33 (Display._c_forceCanvas)

kasmweb/core/display.js

@@ -13,7 +13,6 @@ import { supportsImageMetadata } from './util/browser.js';
 export default class Display {
     constructor(target) {
         this._drawCtx = null;
-        this._c_forceCanvas = false;
 
         this._renderQ = [];  // queue drawing actions for in-oder rendering
         this._flushing = false;