Don't force when generating background migration in housekeeper

What does this MR do and why?

This force: true option seems to generate the migration even if there is an existing migration with the exact same name. Additionally it deletes the existing migration with the same name. This is probably not something we ever intended to do so I'm removing it.

At some point we may want to update this to skip finalizing a BBM if a migration with a matching name exists (or generate a guaranteed unique name) but it's not clear to me if we actually have a use case for this now. Also there are plans to be more strict about not re-using batched background migrations so possibly the matching name won't be very common in future.

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