Remove UserTree implementation
Summary
Since !641 (merged) is now implementing client's persistence thru a SwitcherIOServer Python UserTree re-implementation, libswitcher C++ UserTree feature should be removed for maintenance reasons.
How would you like the feature to work ?
Remove all UserTree from libswitcher.
Why is this feature important to you ?
This is also important as to simplify Switcher's Quiddity model. Quiddities already have an InfoTree to store their state, having a separate UserTree only makes it harder for new developers to understand.