Bug 1440253 - Remove unused nsIDOMCanvasRenderingContext2D. r=IanN

Port bug 1440039 [Remove nsIDOMCanvasRenderingContext2D].

suite/modules/WindowsPreviewPerTab.jsm

@@ -325,14 +325,6 @@ PreviewController.prototype = {
   }
 };
 
-XPCOMUtils.defineLazyGetter(PreviewController.prototype, "canvasPreviewFlags",
-  function () { let canvasInterface = Ci.nsIDOMCanvasRenderingContext2D;
-                return canvasInterface.DRAWWINDOW_DRAW_VIEW
-                     | canvasInterface.DRAWWINDOW_DRAW_CARET
-                     | canvasInterface.DRAWWINDOW_ASYNC_DECODE_IMAGES
-                     | canvasInterface.DRAWWINDOW_DO_NOT_FLUSH;
-});
-
 ////////////////////////////////////////////////////////////////////////////////
 //// TabWindow