Migrate remaining SpecialSeedGenerators to SeedingLayerSetsHits

Migrate SpecialSeedGenerators still using SeedingLayerSetsBuilder to use SeedingLayerSetsHits. These generators are
- BeamHaloPairGenerator
- GenericPairGenerator
- GenericTripletGenerator
- SimpleCosmicBONSeeder

The migration allows removal of ```TrackingRegion::hits(..., const ctfseeding::SeedingLayer*)``` method since nobody calls it anymore, simplifying the TrackingRecHit rework.

There should be no effect on physics.