Remove the extra call to ST_Graph (#240)

I was calling it an extra time when I created the cormen graph, but I
don't need the cormen graph for the st_graph tests!
Merge pull request #246 from agouge/240

Remove the extra call to ST_Graph (#240)

This explains the extra call to `ST_Graph` in #240 ... it's my stupid mistake.

I was calling it an extra time when I created the cormen graph, but I
don't need the cormen graph for the `ST_Graph` tests!