examples: fix duplication (#2813)

Author: niklasvh

examples/existing_canvas.html

@@ -29,8 +29,6 @@ <h1>Existing canvas:</h1>
     var canvas = document.querySelector("canvas");
     var ctx = canvas.getContext("2d");
 
-    var ctx = canvas.getContext('2d');
-
     ctx.beginPath();
     ctx.arc(75,75,50,0,Math.PI*2,true); // Outer circle
     ctx.moveTo(110,75);