Revert "Reduce XP for empty enchants. Fixes BUKKIT-5557"

This reverts commit 9fe4d4f7699f5200c730d40d2a85c0abdad43290.
Revert "[Bleeding] Correctly enchant books. Fixes BUKKIT-5302"

This reverts commit ac1a2d0233eff169efcc7c807cbf799b57bf2306.

I'm just going to revert this change and re-implement the
part that was supposed to allow multiple enchantments on
books. The rest of this commit introduced unintented
side-effects, in that XP costs are no longer deducted
for empty enchant lists.
Allow multiple enchants on books. Fixes BUKKIT-5302
Always deduct XP when enchanting an item. Fixes BUKKIT-5557

The previous behavior was to only deduct XP if enchanting a non-book,
or only when en enchantment as actually added to a book.

The behavior after commit ac1a2d0233 was that XP was not deducted for
items or books if an enchantment was not added.

I have chosen to make the consistent behavior that XP is always
deducted, though if this is not desirable, it would be preferable to
restore the old behavior for non-book items, at least.