Remove unused CanvasGraphics properties (PR 700 follow-up)

These are probably a copy-and-paste mistake, since they appear to have been unused already in PR 700 all the way back in 2011.

Author: Snuffleupagus

src/display/canvas.js

@@ -667,8 +667,6 @@ class CanvasGraphics {
     this.stateStack = [];
     this.pendingClip = null;
     this.pendingEOFill = false;
-    this.res = null;
-    this.xobjs = null;
     this.commonObjs = commonObjs;
     this.objs = objs;
     this.canvasFactory = canvasFactory;