Remove or Expose gMapBaseZ

[duncanspumpkin]

#21370 moved gMapBaseZ into the gamestate structure. But nothing ever sets this to a value other than 7 (except possibly from an s6 file). This issue is to discuss if we should either:

1. Change it to a constant 7 or
2. Expose it in the Ui so users can modify it.

What does gMapBaseZ do:

It controls which land height counts as 0 metres/feet. This can be seen when viewing height labels and watching the altitude graph for a roller coaster. Purely cosmetic.

[Gymnasiast]

Having given it some thought, I can agree to removing it. A constant would do the job well, and the feature isn’t that useful. Our codebase is big enough already.