Allow custom liquids to have drops

The falling node handler previously checked to see if the replaced node was a liquid, and didn't drop any items if it was. THis was a hacky way to make sure the liquids in minetest_game didn't drop water and lava items when filled in.

This has now been fixed in minetest_game by setting the drops property. By removing the builtin check, we now allow user-defined liquids to have a drop if wanted.