Respond to user deletion.
This hook is invoked from user_delete_multiple() before field_attach_delete() is called and before users are actually removed from the database.
Modules should additionally implement hook_user_cancel() to process stored user data for other account cancellation methods.
$account: The account that is being deleted.
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_user_delete($account) {
db_delete('mytable')
->condition('uid', $account->uid)
->execute();
}