[developers] LKB - retains redundant edges in type hierarchy

[Ann Copestake]

glenn at thai-language.com said:
> I expected a graph with 10 edges, rather than 12, since ̵6;a̵7; already
> subsumes both ̵6;f̵7; and ̵6;e̵7; via ̵6;glbtype1̵7; and ̵6;glbtype2̵7;.

yes, you're quite right.  I think that must be a very long-standing issue 
because the relevant file (main/checktypes.lsp) hasn't been touched for 
several years and I'm not sure how quickly it's likely to get fixed.