Remove redundant access check

This access check was causing the upgrade_unattended script to fail.
The check is redundant, since the only product code has ensure access for the
same access level.

core/plugin_api.php

@@ -670,8 +670,6 @@ function plugin_is_installed( $p_basename ) {
  * @return null
  */
 function plugin_install( MantisPlugin $p_plugin ) {
-	access_ensure_global_level( config_get_global( 'manage_plugin_threshold' ) );
-
 	if( plugin_is_installed( $p_plugin->basename ) ) {
 		error_parameters( $p_plugin->basename );
 		trigger_error( ERROR_PLUGIN_ALREADY_INSTALLED, WARNING );