Fix minetest.clear_* creating new LOCAL table instead of clearing the existing one.

Fix minetest.clear_* creating new LOCAL table instead of clearing the existing one.

On calling clear_redistered_biomes the registered_biomes table is cleared
by creating a new empty table, but the pointer is not updated to point to
the new one. So after calling more register_biome, the registered_biome
table always contains 0 items, which is an error. Instead, the table is
cleared by removing all its items so the pointer (minetest.registered_*)
remains valid.