Remove user_preferences.home_organization

There's a home_organization in user_preferences. This was from the old roadmap where a user would be a member of many Organizations and pick one to be their home organization. Now a home organization is a non-optional definition of which Organization owns the user.

Remove the user_preferences.home_organization and related functionality.

This will clear the way for the User.home_organization defined by #546559 (closed)