Cavegen: Fix variable typo that broke mgvalleys large cave distribution

[paramat]

Fixes #7244 (comment)
'max_stone_y' did not match the function argument 'max_stone_height', see the same line in CavesV6::makeCave() which does match the function argument:

minetest/src/mapgen/cavegen.cpp

Line 676 in 574dab5

route_y_max = -of.Y + max_stone_height + max_tunnel_diameter / 2 + 7;

max_stone_y still worked and didn't cause a compile error somehow, maybe because it is present here:

minetest/src/mapgen/cavegen.h

Line 137 in 574dab5

s16 max_stone_y;

And here:

minetest/src/mapgen/mapgen.cpp

Line 827 in 574dab5

void MapgenBasic::generateCaves(s16 max_stone_y, s16 large_cave_depth)

So mgvalleys large caves were broken all this time.

[paramat]

This fix was made to CavesV6() in a605d69 May2016

-	route_y_max = -of.Y + max_stone_y + max_tunnel_diameter / 2 + 7;
+	route_y_max = -of.Y + max_stone_height + max_tunnel_diameter / 2 + 7;

But wasn't done in the general large caves code CavesRandomWalk::makeCave() at that time.

The error has actually been present since 03868ff April 2013
Here in 0.4.6 it was correct, the function argument was 'max_stone_y':

minetest/src/mapgen_v6.cpp

Line 948 in f85c116

void MapgenV6::generateCaves(int max_stone_y) {

minetest/src/mapgen_v6.cpp

Line 1001 in f85c116

s16 route_y_max = -of.Y + max_stone_y + cave.max_tunnel_diameter/2 + 7;

[paramat]

Mgv7 tested and fine.

[paramat]

Updated to remove unused variable from header file.