Clean up DAGScheduler's getMissingParentStages() and stageDependsOn() methods

DAGScheduler has getMissingParentStages() and stageDependsOn() methods, which are suspiciously similar to getParentStages(). All of these methods perform traversal of the RDD / Stage graph to inspect parent stages. We can remove both of these methods, though: the set of parent stages is known when a Stage instance is constructed and is already stored in Stage.parents, so we can just check for missing stages by looking for unavailable stages in Stage.parents. Similarly, we can determine whether one stage depends on another by searching Stage.parents rather than performing the entire graph traversal from scratch.