Dont retract pep items on UserXEPs

This leads to multiple problems

1. We cant assume only items with id='current' are stored in the node
which would lead to retracting 'current' but another item would become
the last published and sent to users

2. Even if we have a SingletonNode retracting the only item means the Node
would be empty and offline clients would not receive the last published item
on coming online, because there is no item anymore

Instead we always publish an empty item from now on