Multisite: Revert [38388].

Restore `get_current_site()` to a multisite only function. Providing this in single site may be a possibility in the future, but should have a dedicated ticket and discussion.

See #37699.


git-svn-id: https://develop.svn.wordpress.org/trunk@38636 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-blogs.php

@@ -1088,11 +1088,13 @@ function get_networks( $args = array() ) {
  *
  * @since 4.6.0
  *
+ * @global WP_Network $current_site
+ *
  * @param WP_Network|int|null $network Optional. Network to retrieve. Default is the current network.
  * @return WP_Network|null The network object or null if not found.
  */
 function get_network( $network = null ) {
-	$current_site = get_current_site();
+	global $current_site;
 	if ( empty( $network ) && isset( $current_site ) ) {
 		$network = $current_site;
 	}