Clear out the user's current group if the associated record is missing.

https://bugzilla.redhat.com/show_bug.cgi?id=1145794

Currently, if the MiqGroup associated through User#current_group is destroyed,
the miq_groups row is removed.

User#current_group_id is non-NULL because we're not clearing it out.
User#current_group is nil because the row with that id is not found.

The fix in the migration cleans up any user's with current_group_id that have non-NULL values that also do not exist in the miq_groups table.
Simplify migration spec by removing join table stub.

https://bugzilla.redhat.com/show_bug.cgi?id=1145794