Deprecate hoes to follow changes in Minetest Game

Hoes crafted from "rare" materials were deprecated in
https://github.com/minetest/minetest_game/commit/9c459e77ac943f10c74de3bdd420e07a2b8bb56f.

Hoes are still available in existing worlds, but they cannot
be crafted anymore.

This closes #29.