Cleanup register artifacts

- Remove mod/register
- Remove REGISTER_* constants

boot.php

@@ -97,19 +97,6 @@ define('SSL_POLICY_FULL',         1);
 define('SSL_POLICY_SELFSIGN',     2);
 /* @}*/
 
-/**
- * @name Register
- *
- * Registration policies
- * @{
- */
-define('REGISTER_CLOSED',        \Friendica\Module\Register::CLOSED);
-define('REGISTER_APPROVE',       \Friendica\Module\Register::APPROVE);
-define('REGISTER_OPEN',          \Friendica\Module\Register::OPEN);
-/**
- * @}
-*/
-
 /**
  * @name CP
  *