[Rebuild] Stray nodes

[Paul Norman]

Some recent edits have made me realize two potential issues that should be
considered when removing data

1. Stray nodes

Suppose a decliner creates a way then an acceptor refines the geometry by
adding new untagged nodes to the way. 

In this case, the way will need to be removed but the acceptor's nodes are
not, however I believe they should be. If not, there will be a massive
number of stray nodes created by the process.

The logic I suggest is that untagged nodes that are children of an object
removed and not children of any other objects also be removed.

2. Removal of some nodes from areas may result in self-intersecting
geometries. I'm not sure how these could be dealt with.