Remove `_sound_def` support method

[OgelGames]

Reasoning:

• Redundant - XCompat has no dependencies, and already optionally depends on games and mods, so why would a mod ever need to optionally depend on XCompat?
• Hacky - It requires overriding minetest.register_node to support nodes that are added after mod loading. (see init.lua#L19-L36
• Confusing - Reading the code of a mod that uses it, it's not clear what _sound_def is used for.
• Unused - The only mods that use it (as far as I could find) are mt-mods/pipeworks, mt-mods/homedecor_modpack, and mt-mods/girder.