Caverns: Remove unnecessary liquid excavation

Caverns: Remove unnecessary liquid excavation

Also disable CavesRandomWalk at a safer distance from caverns.

Excavating liquids in cavern code is unnecessary as in practice we are already
successfully disabling the generation of liquid caves that could intersect
with caverns and cause excessive amounts of spreading liquids in caverns.

However to be safer this commit now disables liquid caves at a larger distance
from caverns, to compensate for liquid caves being able to generate up to a
mapblock beyond a mapchunk border.

Not excavating liquids in cavern code also allows a feature i am working on in
experimental new core mapgens, but also allows for more flexibility in future.