Remove limitation [15, 85] for landmass

[Corbeau-1]

Currently it is impossible to generate map that would contain 0% or 100% land. The limit is set to 15% and 85%. Please remove it.

I presume there may be generator crashes, however, this is up to the ruleset maker to solve, that is, to not set up such numbers to cause the crash.

Reason: creating some exotic tile/rulesets that require some more flexibility.