Replace deprecated `background-image-svg()` mixin calls

Use normal `background-image` properties with SVGs only now
that IE 8 and Android 2.1 is removed from Grade C.
Also removing all PNG fallback images.

Bug: T248062
Change-Id: Ib91cd0514d331ab6a0f8b668aef6991cf3267fe2

resources/skins.vector.styles/Menu.less

@@ -40,9 +40,7 @@
 /* Icon for registered user names & anonymous message */
 #pt-anonuserpage,
 #pt-userpage a {
-	// SVG support using a transparent gradient to guarantee cross-browser
-	// compatibility (browsers able to understand gradient syntax support also SVG)
-	.background-image-svg( 'images/user-avatar.svg', 'images/user-avatar.png' );
+	background-image: url( images/user-avatar.svg );
 	background-position: @background-position-nav-personal-icon;
 	background-repeat: no-repeat;
 	background-size: @background-size-nav-personal-icon @background-size-nav-personal-icon;

resources/skins.vector.styles/MenuDropdown.less

@@ -23,7 +23,7 @@
 
 		&:after {
 			content: '';
-			.background-image-svg( 'images/arrow-down.svg', 'images/arrow-down.png' );
+			background-image: url( images/arrow-down.svg );
 			background-position: 100% 50%;
 			background-repeat: no-repeat;
 			position: absolute;

resources/skins.vector.styles/SearchBox.less

@@ -96,7 +96,7 @@
 }
 
 #searchButton {
-	.background-image-svg( 'images/search.svg', 'images/search.png' );
+	background-image: url( images/search.svg );
 	background-position: center center;
 	background-repeat: no-repeat;
 }

resources/skins.vector.styles/common/typography.less

@@ -129,9 +129,9 @@ pre,
 // External links
 .mw-parser-output {
 	.external {
+		background-image: url( images/external-link-ltr-icon.svg );
 		background-position: center right;
 		background-repeat: no-repeat;
-		.background-image-svg( 'images/external-link-ltr-icon.svg', 'images/external-link-ltr-icon.png' );
 		padding-right: 13px;
 	}
 }