Revert "wp++"

This reverts commit 89fcc36fb1.

wp-admin/network/plugin-install.php

@@ -13,4 +13,7 @@ if ( isset( $_GET['tab'] ) && ( 'plugin-information' == $_GET['tab'] ) )
 /** Load WordPress Administration Bootstrap */
 require_once( dirname( __FILE__ ) . '/admin.php' );
 
+if ( ! is_multisite() )
+	wp_die( __( 'Multisite support is not enabled.' ) );
+
 require( ABSPATH . 'wp-admin/plugin-install.php' );