[worker] Remove unused var. (#195)

Author: ovvn

src/worker.js

@@ -176,7 +176,6 @@ Worker.prototype.toPdf = function toPdf() {
     var opt = this.opt;
 
     // Calculate the number of pages.
-    var ctx = canvas.getContext('2d');
     var pxFullHeight = canvas.height;
     var pxPageHeight = Math.floor(canvas.width * this.prop.pageSize.inner.ratio);
     var nPages = Math.ceil(pxFullHeight / pxPageHeight);