Don't delete account_id row on MySQL

If the table is an InnoDB table deleting the row after allocation
may cause the sequence to reset when the server restarts, giving
out duplicate account_ids later.

Change-Id: I3e81df92a2a09c2024e047d5eef349f6524f83db

gerrit-reviewdb/src/main/resources/com/google/gerrit/reviewdb/mysql_nextval.sql

@@ -9,7 +9,6 @@ CREATE FUNCTION nextval_account_id ()
   MODIFIES SQL DATA
 BEGIN
   INSERT INTO account_id (s) VALUES (NULL);
-  DELETE FROM account_id WHERE s = LAST_INSERT_ID();
   RETURN LAST_INSERT_ID();
 END;
 //