Badpenny: remove `result` from jobs

Jobs have a `result` column that badpenny clients can use to store results from a job that are more detailed than "succeeded" or "failed".

@Callek pointed out that the column is a generic JSON blob, and that using such things in a DB has gotten us into trouble before.

I don't have any practical use for this column.  Most jobs either won't need any results (cleanups, query-and-send-email, etc.) or will need to store results in a more complex fashion (slaveloan).

So let's deep-six it.