Don't use BuildingAI for ships

[elexis]

Since ships were introduced they used BuildingAI to shoot arrows when being garrisoned and use UnitAI to have a stance, move around and get into attack range.

This resulted in inconsistencies like 1 arrow being shot from UnitAI and N arrows being shot from BuildingAI.

The code to determine the number of garrisoned arrows should be refactored so that both buildings and ships can call the same methods to shoot arrows without ships using BuildingAI logic to f.e. check for enemies in range.