Remove group awareness on new tree node creation

Groups are a relict of the object tree. Since the annotation system aims to
replacing the object tree, there is no need to take care on the group a tree
node is in.
Remove group awareness when giving neuron to other user

The annotation system replaces the object tree. Therefore groups are not needed
anymore.
Make delete_if_empty more readable and efficient

For an existence test EXISTS is usually better and faster than COUNT(*).
Remove IST group test on tree node deletion

Annotations replace the object tree and therefore groups. When deleting tree
nodes it is therefore not required anymore to check if a node is part of the
"Isolated Synaptic Terminals" group.
Annotations: fix split annotation assignment
Skeleton merging: delete the losing neuron