Experience orbs don't drop instantly on death

Summary:

Mobs should drop experience points the moment they are killed, rather than when their corpse despawns. This is a parity difference introduce in Java snapshot 19w46a

Steps to reproduce:

1. Find the nearest mob.
2. Kill it via a method that would register as a player's kill.

Observed results:

While loot drops immediately, experience points aren't dropped until the mob's corpse has despawned.

Expected results:

Experience points should drop immediately along with loot, matching Java behavior from snapshot 19w46a onwards.