Removing the leaves check for stockpile acceptability.

That item type no longer exists in DF 0.40; such items will now always be considered properly stored by any stockpile they're on.
Removing an obsolete workaround
Speeding up the stockpile item count.

kr0pper reports two orders of magnitude speed improvement for check_pile() by using the map tile item cache.
Merge pull request #299 from eswald/stockflow-40

Stockflow Repairs