online-drat-check: make sure units can be removed
online-drat-check: make sure units can be removed

When removing units from the proof, the proof checker might trigger an
assertion, because the index is compared to a size, but a pop operation
is also executed. Introduce an additional bool to indicate that a clause
has actually been removed from the proof.

nmanthey/riss-solver,nmanthey/riss-solver,nmanthey/riss-solver