quadtree: Be more careful when deleting a node from the tree

quadtree: Be more careful when deleting a node from the tree

Before attempting to move nodes back to the main tree, detach the
subtree completely from the main tree. When detaching individual nodes
from the tree, also ensure that each node has its old children
removed.

If these measures are not taken into account, loops will form in the
tree.

Signed-off-by: Timo Kokkonen <kaapeli@itanic.dy.fi>