Remove `.background-size()` mixin usage

Removing deprecated LESS `.background-size()` mixin calls.
Also
- simplifying code, where second (height) value is automatically set
  to `auto` when only one value is set and
- removing stylelint rule.

Bug: T219956
Change-Id: I643c66b3067ba77726d76cbebf7550781e18aa5d

.stylelintrc.json

@@ -2,9 +2,6 @@
 	"extends": "stylelint-config-wikimedia",
 	"rules": {
 		"declaration-no-important": null,
-		"property-blacklist": [
-			"background-size"
-		],
 		"selector-list-comma-newline-after": null,
 		"selector-max-id": null,
 		"value-keyword-case": null

resources/skins.minerva.base.styles/ui.less

@@ -152,7 +152,7 @@ main {
 	background-color: #fff; // Support Fennec, Opera Mini: Remove default background, see T38490.
 	background-position: left @searchIconGutter center;
 	background-repeat: no-repeat;
-	.background-size( @searchIconSize, @searchIconSize );
+	background-size: @searchIconSize;
 	// Support Safari: Get rid of rounded corners.
 	-webkit-appearance: none;
 	width: 100%;

resources/skins.minerva.content.styles/links.less

@@ -45,7 +45,7 @@ a {
 	&.external {
 		// background-image is specified by ResourceLoader.
 		// Force the image size to be 10px.
-		.background-size( 10px, 10px );
+		background-size: 10px;
 		background-repeat: no-repeat;
 		background-position: center right;
 		padding-right: 13px;

resources/skins.minerva.content.styles/templates/ambox.less

@@ -104,7 +104,7 @@ table.ambox {
 	}
 
 	.mw-ui-icon:before {
-		.background-size(75%, auto);
+		background-size: 75%;
 	}
 
 	.ambox-learn-more {

skinStyles/mobile.startup/toast.less

@@ -70,7 +70,7 @@
 		// allows extensions ot use mw.notify( msg, 'error' ) and have those styled as errors.
 		// see https://github.com/search?l=JavaScript&q=org%3Awikimedia+mw.notify&type=Code
 		.background-image( 'images/error.svg' );
-		.background-size( 24px, 24px );
+		background-size: 24px;
 		background-position: 16px 50%;
 		background-repeat: no-repeat;
 		padding-left: 5%;