Skeleton Horse underwater step and gallop sound events are unused

The sound events for Skeleton Horses galloping and stepping, "minecraft:entity.skeleton_horse.gallop" and "minecraft:entity.skeleton_horse.step", are played regardless of whether or not the Skeleton Horse is in water.

This leaves the sound events, "minecraft:entity.skeleton_horse.gallop_water" and "minecraft:entity.skeleton_horse.step_water" unused.