minecraft:transformation component into xp_orb doesn't remove the source entity

Partner Rank: 2

Verification builds: 1.13.0.15
Regression builds: 1.12

Summary: When a mob transforms into an xp_orb, it sticks around (and is therefore free to transform again, creating infinite orbs).

Impact: This breaks existing content ANIMALS, causing infinite XP lag spam when you kill a mob.

Repro Steps:

1. Download and open the attached world.
2. Press the button on the left. It spawns a pig with behaviors to transform into a cow after one second.
3. Press the button on the right. It spawns a sheep with identical behaviors to transform into an xp orb after one second.

Expected Results: Both mobs transform the same way.

Observed Results: Sheep is not removed, continues to spawn infinite XP orbs.

Screenshots/Videos attached: Yes