WordPress Walker — move custom menu class from list item to link

functions.php

<?php

class Social_Walker extends Walker_Nav_Menu {
	function start_el(&$output, $item, $depth, $args) {
		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		$link_class = $classes[0];
		$link_target = $item->target == '' ? '' : 'target="'.$item->target.'"';
		unset($classes[0]);

		$output .= '<li class="'. join($classes, ' ') .'">'
			.$indent.'<a href="'.$item->url.'" title="'.$item->title.'" class="'.$link_class.'" '.$link_target.'>'
			.$indent.'<span class="sr-only">'.$item->title.'</span>'
			.$indent.'</a>';
	}
}