Remove offline pruner once online pruner is fast enough

[qdm12]

The main reason the offline pruner is kept is because online pruning has an impact on performance.
The offline pruning does add some complexity in the code and even ourselves wondered for a long time why it was here.

Once #2854 is done, we should test the performance of the node with and without online pruning.
If impact is (as I would expect) minimal, we can remove the offline pruning feature/code.