Deprecate and remove use of the 'SUBSTRING_FOR_DATE' constant.

With MySQL support removed (b154bc7a10) this constant is unnecessary.

classes/Pref_Users.php

@@ -24,11 +24,11 @@ class Pref_Users extends Handler_Administrative {
 			$id = (int) clean($_REQUEST["id"]);
 
 			$sth = $this->pdo->prepare("SELECT login,
-				".SUBSTRING_FOR_DATE."(last_login,1,16) AS last_login,
+				SUBSTRING_FOR_DATE(last_login,1,16) AS last_login,
 				access_level,
 				(SELECT COUNT(int_id) FROM ttrss_user_entries
 					WHERE owner_uid = id) AS stored_articles,
-				".SUBSTRING_FOR_DATE."(created,1,16) AS created
+				SUBSTRING_FOR_DATE(created,1,16) AS created
 				FROM ttrss_users
 				WHERE id = ?");
 			$sth->execute([$id]);