Index: wp-admin/includes/plugin.php
===================================================================
--- wp-admin/includes/plugin.php	(revision 13871)
+++ wp-admin/includes/plugin.php	(working copy)
@@ -479,7 +479,6 @@
 			$output = ob_get_clean();
 			return new WP_Error('unexpected_output', __('The plugin generated unexpected output.'), $output);
 		}
-		do_action( 'activate_plugin', trim( $plugin) );
 		if ( $network_wide ) {
 			$current[$plugin] = time();
 			update_site_option( 'active_sitewide_plugins', $current );