I would guess they've been chipped away by over-zealous religious types who were trying to reduce levels of idolotry in the church. We've had similar instances in the UK where the heads are hacked off statues or whatever. The reformation never took hold here, but I can't help thinking that the pre-dominance of white-washed interiors in the churches reflects a similar attitude regarding imagery and painting.

But balance this against the richly decorated Baroque interior of the main church building... The jury is probably still out.