don't leak GNodes in plist_free

Before recursing over its children, plist_free_node started by
detaching the current GNode from its parent which means that
calling g_node_destroy on the root of the tree was freeing only
the top-level GNode while what was intended was to free the whole
tree. Don't leak memory by not detaching children GNodes from their
parents so that g_node_destroy on the toplevel GNode can clean
everything.
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