 Delete-relaxation heuristic

For positive-precondition STRIPS with a positive conjunctive goal, remove every delete effect and use the exact shortest relaxed plan cost. Every original plan still succeeds in the relaxed model because facts can only accumulate, so its relaxed optimum is an admissible heuristic. The cost of an arbitrary relaxed plan is an upper bound on the relaxed optimum and need not be an admissible heuristic for the original problem.