Phantoms can spawn above mushroom islands / mushroom fields

The bug

The issue is that the new phantom mob spawns above mushroom islands, even though no other hostile mob can do the same. This contradicts the previous spawning system.

Through testing on multiple superflat worlds of various biomes, I have found that phantoms spawn over mushroom islands, along with every other overworld biome.

This isn't much of an issue for most, but if anyone was to set up a base on a mushroom island, this harms the very concept of it being a location without any hostile mobs.

This should be an easy fix since they have already been removed from the end. I'm guessing it was overlooked because they have their own spawning system completely independent from other hostile mobs.

Code analysis

Code analysis can be found in this comment.