Remove image type description when saving to png (#566)

The description is the name of the file type in the dialog. This shouldn't be Excalidraw but the default one for a png.

src/scene/data.ts

@@ -227,7 +227,6 @@ export async function exportCanvas(
       if (blob) {
         await fileSave(blob, {
           fileName: fileName,
-          description: "Excalidraw image",
         });
       }
     });