Fix pruning of covering intervals in proofs (#8084)

Fix pruning of covering intervals in proofs (#8084)

We need to mirror the pruning of intervals in the coverings solver in the corresponding proof. To match the proof nodes with the coverings-internal intervals, we use ids, where non-interval nodes have the id zero. In this process, we had three separate issues that never showed up because we don't have a proof checker for CAD proofs:

we did not check for zeros, pruning all non-interval nodes
we ran the pruning on the direct children of a CAD_RECURSIVE proof step, which are SCOPE nodes that don't have ids
iterating over and then removing children was done by a custom but faulty reimplementation of std::remove_if
Thus, we now

never prune zero nodes,
prune based on the (single) child node, if we are checking a SCOPE node, and
simply use std::remove_if.