Wither's nether star drop is hardcoded

Issue:
The wither's loot table is still hard-coded, despite most other loot tables being data driven. This makes it impossible for a datapack developer to modify or remove it's nether star drop.

How to replicate:
Open the game's default datapack, then open the loot table located at entities/wither. It will not contain an entry for a guaranteed nether star

Expected behavior:
The loot table would match with the wither's drops.