Randomwalk cave liquids: Remove deprecated 'lava depth' parameter

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Randomwalk cave liquids: Remove deprecated 'lava depth' parameters

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'.
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From MT 5.0.0 it has been possible to define and locate cave liquids using biome definitions, and this is the intended method for the future. The hardcoded cave liquid code will eventually be removed.

The 'lava depth' parameter has been deprecated since MT 5.0.0, this PR removes it as a first low-disruption step towards the removal of the hardcoded cave liquid code. This will also encourage users to use biome definitions to locate lava.

If cave liquids are not defined and located using biome definitions (games other than MTGame):

• In MT master, lava appears only below 'lava depth', which by default is y = -256 for most mapgens, y = 1 in mgvalleys.
• With this PR, lava appears only below y = water_level - 256, so no change for most mapgens using the default 'lava depth'.

Note that MTGame already uses biome definitions to define and locate cave liquids, so there will be no changes for anyone using MTGame.

If merged, i will make a detailed post in the news subforum to warn of the change in case anyone will be significantly affected.
If anyone has to alter code, this will just be bringing forward the inevitable, as everyone will have to define and locate cave liquids in biome definitions when the horrible hardcoded cave liquid code is completely removed in future.

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Now complete, tested and ready.
PR may look big but changes are simple and duplicated across 6 mapgens.

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The 'lava depth' parameter was added in June 2017 in 2652d8d
I see in that commit that lava depth used to be hardcoded to y = -256, so alternatively i could change the new upper limit for lava to 'water level - 256'. This would mean no change for most users and would be consistent with classic behaviour.
EDIT: Will do this.
EDIT: Done.

[paramat]

This parameter is very young, it was added after release of 0.4.16, not present in 0.4.17, released into stable in 5.0.0, so has only been present in 1 stable release, it will not be in much use. This even further reduces the chance of disruption.

[paramat]

Huh ... just realised something ... the 'lava depth' parameter was released into MT 5.0.0, but in MT 5.0.0 the harcoded cave liquid code was already deprecated. So the 'lava depth' parameter should never have been released into MT 5.0.0., it should have been removed before release.
My mistake, sorry for that. Even more good reason to remove this parameter as soon as possible.