DB migrations is in a bad state

The DB migrations in master (and 2.7) are in a bad state. If you try to generate them in a clean tree, you get this:

  Migrations for 'maasserver':
  src/maasserver/migrations/maasserver/0202_auto_20200207_1516.py
    - Alter field node on event

Tracking it down, it looks like d593bba28ede53bc5215f65df02bbe8f2f72d0e3 introduce a DB migration without a corresponding change in the Python code.

It looks like we want it like the Python code currently defines it. Commit
1252f885bd changed it to DO_NOTHING to make Node.delete() faster.

So we need to add a DB patch, rather than change the Python code.