D3269 / rP24635 made mercenaries unable to gather resources, though infantry can still build.
D3290 / rP25174 introduced a Builder visible class for units that can build structures.
This patch removes the Worker class from mercenary infantry (cavalry and champions don't have it anyway), which may confuse players, because it suggests they could gather.
Mercenaries keep the Citizen and CitizenSoldier classes for now, because the AI needs those for training units.
While at it, the Mercenary class is removed from the Persian kardakes champions, which are not trainable.