Rework mob item dropping on death. Fixes BUKKIT-5625

After the changes in d611cff2 we started including a mob's equipment when
calling EntityDeathEvent so plugins can access this data. However, the
changes to enable this triggered a bug that makes skeletons and pig zombies
no longer drop equipment because they handle this differently than the rest.
On top of this we don't handle dropping equipment for mobs that cannot
pick up items in vanilla even though vanilla does drop equipment for them
if you summon them with it. We also do not include a horse's inventory
in the event so they drop their saddle, armor, chest, and chest contents
with no way for a plugin to control this.

To solve this issues we revert mob item dropping back to vanilla logic
and instead just capture all their drops in the method they all call to
spawn them into the world. We also move horse inventory dropping so it
happens at a time when we're capturing these drops. With these changes
all items mobs drop on death should now be included in the event and
we have less diff to worry about for future updates.