[C] Fix equipment drops for skeletons and pig zombies. Fixes BUKKIT-5625

The Issue:
Commit https://github.com/Bukkit/CraftBukkit/commit/d611cff2b37e817c1070b83aeb93a79cd8662bc2 changed the way equipment drops are calculated. Since then skeletons and pig zombies don't drop any of their equipment when they die.

Justification for this PR:
Skeleton and pig zombies should drop their equipment like in vanilla Minecraft.

PR Breakdown:
https://github.com/Bukkit/CraftBukkit/commit/d611cff2b37e817c1070b83aeb93a79cd8662bc2 changed the code in EntityInsentient. Skeletons can drop more than one item. The first is arrow or coal, the second is bone. So EntitySkeleton don't call the super method from EntityInsentient and the code for dropping equipment is missing. This PR adds the same code as in EntityInsentient to fix the issue.
This also applies to pig zombies. Other entities type don't support equipment, so this bug don't apply to them. These are: cow, chicken, sheep, pig, ocelot, magma cube, iron golem, ghast, enderman and creeper.

Testing Results and Materials:
Before this PR no skeletons or pig zombie will drop any equipment like bows. After applying this PR the drops of skeletons and pig zombies are working as intended again.

Relevant PR:
(none)

JIRA Ticket:
BUKKIT-5625 - https://bukkit.atlassian.net/browse/BUKKIT-5625