drop backward compatibilty import for migrate < 0.6.1

We don't suport sqlalchemy-migrate older then 0.6.1. All exceptions
where moved to migrate.exception in 0.6.1 release. So no need trying
to import exceptions by the old name.

This fixes pylint 'no-name-in-module' error for the removed code.
drop monkey patch for old migrate bug

The versions of sqlalchemy-migrate that we support, does not
have this bug. No need for monkey patching.

This fixes pylint 'no-name-in-module' error for the removed code.