RemoveCounterCost could use improvements in target selection

The RemoveCounterCost class (used in VOW by Hopeful Initiate) could use some improvements in selecting targets to remove counters from.

- canPay only checks if there are valid targets on the board (so any creature on the board means the ability highlights as available to use even if there are not enough counters to pay).
- It allows the player to target a creature with no counters.
- May want to use the multi-target UI from #7528 when there is more than one target

This doesn't actually break gameplay. The pay function works correctly and it just backs out if there's not enough counters among targeted creatures but it would be a nice QOL to fix this up.