Treenode table: remove last skeleton functionality

This functionality isn't needed anymore. These widgets can now be opened from
the neuron navigator and not only for the currently active node. Given that the
treenode table isn't used very much, it does not need to remember the last
skeletons.
Connector table: remove last skeleton functionality

This functionality isn't needed anymore. These widgets can now be opened from
the neuron navigator and not only for the currently active node. Given that the
connector table isn't used very much, it does not need to remember the last
skeletons.