Refactoring MustFightBattle

[Trevan]

I've been working on the offensive subs submerge/withdraw step and discovered that my idea of having a unified valid() check that works for both getNames and execute won't work.

In the offensive subs submerge/withdraw step, execute shouldn't run if there is a destroyer present. But getNames is called at the beginning of the round and there might be a destroyer present that will die before execute is called. If getNames doesn't return the submerge/withdraw step, then an error will occur.

I was actually able to trigger this error from the master branch when Submersible Subs is false, Sub Retreat Before Battle is false, and a destroyer is present at the beginning of the round but then is destroyed during the round. If Submersible Subs is true and Sub Retreat Before Battle is false, then this error doesn't occur because it doesn't check for destroyers.

I'm going to be removing valid() as a public method from the BattleStep interface. Instead, getNames and execute will do their own internal valid check. This check might be the same or different, depending on the step.