Randomwalk cave liquids: Remove deprecated 'lava depth' parameter (#9105)

Randomwalk cave liquids: Remove deprecated 'lava depth' parameter (#9105)

Low-disruption first step towards removing the hardcoded cave liquid
code. Since MT 5.0.0 cave liquids can be defined and located by
biome definitions instead.
In games that do not yet use biome definitions to define and locate
cave liquids (MTGame does), lava will now appear below
y = water_level - 256 instead of below 'lava depth' (usually y = -256).
Therefore no change in most mapgens if using the default 'lava depth'.