fix: DB migration fails with AWS RDS MySQL 5.7

- issue: the "DROP TRIGGER" step of our MySQL DB migration fails
when used with AWS RDS MySQL 5.7 due to:
```
(conn=127) You do not have the SUPER privilege and binary logging is enabled
```
- We decided that the best approach is to just remove
this migration step, even though modifying past migrations is
not recommended by flyway (but in the past, in our context
removing migration steps have not created any problems).
- Note: This removal comes with a small problem where the triggers
in question are not removed (because we do want these triggers
to be removed, since they are not compatible with our latest code in
terms of DB cleanup, see: #728).
However, this problematic scenario should be quite narrow
(only for users who have successfully installed CredHub 2.12.67,
which adds the triggers, and have never upgraded to
any versions between 2.12.69 and the latest current version (2.12.84),
which contains the "DROP TRIGGER" step). For these users, we
will publish docs on how to fix it (manually drop the triggers).

[#187774971]