Unsets a persistent variable.
Case-sensitivity of the variable_* functions depends on the database collation used. To avoid problems, always use lower case for persistent variable names.
$name: The name of the variable to undefine.
function variable_del($name) {
global $conf;
db_delete('variable')
->condition('name', $name)
->execute();
cache('bootstrap')
->delete('variables');
unset($conf[$name]);
}