Player-summoned ender dragon drops dragon egg after server reboot or close/open world

A player summoned ender dragon will drop an egg if alive after a server reboot.

To reproduce:

1. Start a fresh end world

2. destroy all crystals

3. kill dragon

4. take egg

5. place 4 crystals around portal to overworld in end

6. wait for crystals and dragon to finish spawning

7. stop server

8. start server

9. see in console that the dragon has not yet been killed (see below)

10. destroy crystals

11. defeat dragon

12. find 2nd dragon egg above portal

seen in console after first dragon has been defeated, new dragon summoned and server rebooted:

[12:26:02] [Server thread/INFO]: Scanning for legacy world dragon fight...
[12:26:02] [Server thread/INFO]: Found that the dragon has not yet been killed in this world.
[12:26:02] [Server thread/INFO]: Found that there's a dragon still alive (EntityDragon['Ender Dragon'/14210245, l='Dragonz', x=-28.97, y=74.54, z=-19.76])
[12:26:02] [Server thread/INFO]: But we didn't have a portal, let's remove it.