WIP: fix #313 regressed ngrams grouping

It turns out that we don't need to call `pruneForest` inside
`searchtableNgrams`. After all, we are just displaying information that
should be correct in the database from the get-go, so why we would need
any of that?

It turns out that the fix for #313 to support duplicated ngrams during
importing broke hierarchical grouping.

I have just temporarily commented out the code and I need to find a fix,
better also writing a test to ensure that grouping will still work in the
future.