fix merge error in pngstream.js

See #740 (comment)

Author: zbjornson

lib/pngstream.js

@@ -51,23 +51,6 @@ var PNGStream = module.exports = function PNGStream(canvas, sync) {
 
 util.inherits(PNGStream, Readable);
 
-var PNGStream = module.exports = function PNGStream(canvas, sync) {
-  Readable.call(this);
-
-  var self = this;
-  var method = sync
-      ? 'streamPNGSync'
-      : 'streamPNG';
-  this.sync = sync;
-  this.canvas = canvas;
-
-  // TODO: implement async
-  if ('streamPNG' === method) method = 'streamPNGSync';
-  this.method = method;
-};
-
-util.inherits(PNGStream, Readable);
-
 function noop() {}
 
 PNGStream.prototype._read = function _read() {