Amend Base10 color and remove (affected) PNG fallbacks

Amending Base10 to slightly darker `#202122` to fulfill WCAG
requirements in connection to Accent50.
Also removing all PNG fallbacks as they are only used in JS-only.

Bug: T248393
Change-Id: I07fa4e23320763a634e5340ab125634842b86454

resources/css/ext.uls.interlanguage.less

@@ -2,8 +2,7 @@
 @import 'mediawiki.mixins';
 
 #p-lang .uls-settings-trigger {
-	background: transparent no-repeat center top;
-	.background-image-svg( '../images/cog-sprite.svg', '../images/cog-sprite.png' );
+	background: transparent url( ../images/cog-sprite.svg ) no-repeat center top;
 	border: 0;
 	min-height: 16px;
 	min-width: 16px;