Delete Purchase Log via Purchase Log Action API callbacks

For #1576

Deletes a Purchase Log via the Purchase Log Action API callback and deprecated deleting via simple un-nonced URL query.

Also wpsc_delete_purchlog() should now require a purchase log ID to be passed. It previously defaulted to checking the URL query if no parameter was passed, and that method should now no longer be used.
For clarity, can we mention in the doc block a note about where this is nonced and cap-checked elsewhere?  Don't want to be looking at this and thinking "hmm, weird that this function that deletes stuff isn't secure."