Drop PrFTHitHandler for good

the goal is to completely get rid of the the PrFTHitHandler, potentially by using the ModPrSciFiHitsSOA data structure and some explicit vectorisation if that speeds up the copying back and forth currently done when reducing the hits in the Hybrid Seeding.

The following discussion from !3451 (merged) should be addressed:

• @gunther started a discussion: (+15 comments)

  can you remove this and PrHit as well or is it still used?