remove rank entries for continuous spans on import

[thomaskrause]

For each spanning relation (thus a span covering a token) there is an entry in the rank table with the edge. This information is never used in the query generation, but when re-constructing the graph.

For continuous spans the graph can be re-constructed without this explicit storage of the edges (using the left/right_token_index). We should remove the unnecessary rank entries on import.

This will reduce the size of the facts table and it's indexes dramatically on corpora that only contain spans.