Cannot remove `DisjointListsNode` from a model

[arcondello]

In order to call graph::pop_decision() on a DisjointListsNode, we need it to not have any successors. Unfortunately the DisjointListNodes cannot be removed

dwave-optimization/dwave/optimization/include/dwave-optimization/nodes/collections.hpp

Line 286 in fe34fef

bool removable_() const override { return false; }