Re-add hacky fix for underwater grass, to fix mgv7 user's biomes

Fixes TenPlus1's problem as described on IRC http://irc.minetest.ru/minetest/2014-12-18#i_4069533
Removes my problematic code and returns to hmmmm's hacky fix, this re-enables 2 layers of biome material under water. Tested in mgv5 and mgv7.