[Gameplay A24] - Remove Spartan population penalty.

Since they don't have much bonuses (structure health, stronger female citizens) to stand up against two penalties (no stone walls and this).
This removes the population penalty; other civilisations have better bonuses but no such penalties.
Besides, Sparta was larger than other city states in ancient Greece, both in terms of area and army size.

Patch by: @Nescio
Differential revision: D3376
Reviewed by: @borg-