Skeleton Horses are part of the "followable_friendly_mobs" tag, which is inconsistent with other undead mobs

Ghastlings occasionally follow Skeleton Horses as they are part of the "followable_friendly_mobs", this is inconsistent with all other undead mobs, including camel husks and zombie horses.

Steps to reproduce the issue:

• Summon a ghastling near a skeleton horse.

Expected result:

• The ghastling will not follow the skeleton horse.

Actual result:

• The ghastling occasionally does follow the skeleton horse.