Networks and Sites: Remove unused variable in populate_networks().

This commit simply removes a `$network_exists` variable (introduced in r41384) that was never used.

Props mukesh27, Soean.

Fixes #64851.

git-svn-id: https://develop.svn.wordpress.org/trunk@61992 602fd350-edb4-49c9-b593-d223f7449a82

Author: JJJ

src/wp-admin/includes/schema.php

@@ -1033,7 +1033,6 @@ function populate_network( $network_id = 1, $domain = '', $email = '', $site_nam
 	}
 
 	// Check for network collision.
-	$network_exists = false;
 	if ( is_multisite() ) {
 		if ( get_network( $network_id ) ) {
 			$errors->add( 'siteid_exists', __( 'The network already exists.' ) );