Revise relationship between User and LivingLab

[BasMichielsen]

The previous relationship implied an optional n-1 association between User and LivingLab such that a user could have zero or one living lab as "their" living lab. Initially with the idea of being able to mark users as "working at this living lab".
This relationship does not reflect reality as there are users that work at multiple living labs and neither is there a wish for this relationship. The current implementation has been removed, however the database still support it.
Can it be removed from the database too?