Remove unused properties and variables

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

* 9ff86fb718 (RFB._mouse_arr)
* bb6965f2e6 (old_requestAnimationFrame)
* 490d471c53 (Display._c_forceCanvas)

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;

core/rfb.js

@@ -134,7 +134,6 @@ export default class RFB extends EventTargetMixin {
 
         // Mouse state
         this._mouse_buttonMask = 0;
-        this._mouse_arr = [];
         this._viewportDragging = false;
         this._viewportDragPos = {};
         this._viewportHasMoved = false;