Bug 25702: Update Tor Browser icon to follow design guidelines

Updated aboutDialog.css to point to the branded default256.png icon now
stored in the firefox jar. Removed old default256.png from torbutton.

src/chrome/skin/aboutDialog.css

 #leftBox {
-  background-image: url('chrome://torbutton/skin/default256.png');
+  background-image: url('chrome://branding/content/icon256.png');
   background-position: right top;
   background-size: 180px;
 }