Redo cat with only black and white and lose png version

Also made slightly bigger.

Change-Id: I91d41378d30ccb2439d44ccdc2caad646fcb9c37

resources/screen-desktop.less

@@ -14,7 +14,7 @@
 }
 
 #mw-content-container {
-	.background-image-svg( 'images/cat-grey.svg', 'images/cat-grey.png' );
+	.background-image( 'images/cat.svg' );
 	background-repeat: no-repeat;
 	background-position: center 15em;
 }