Delete events without submitter

This will clean up the database.

Fixes https://github.com/openSUSE/osem/issues/329 and https://github.com/openSUSE/osem/issues/324
> If we have records in event_users table for a submitter and a speaker, who is a different user, event.users still
> returns a result even if the record of one of them is gone. So event.users.blank? will be false but 
> event.submitter.blank? will be true.

1. Event.users is a collection proxy. I'm pretty sure it can't collect records that don't exist.
2. Event.submitter will return EventSubmitter.user which is nil if the user doesn't exist
3. both will respond to blank? with true

I'm not saying you should just blindly replace this call...

* submitter is a method(!) in Event that scopes EventUsers to "event_role: submitter"
* speakers is an association to User, in Event, through event_users
* users is an association to User, in Event, through event_users 

We want to get rid of the speaker and submitter *"roles"* in event_users in the future. Hence you should use a temporary Event in the migration that calls what's going to be left.