Remove inheritance from HitTripletGenerator from HitTripletGeneratorFromPairAndLayers

HitTripletGeneratorFromPairAndLayers does not really benefit from the
base class, and the user (CombinedHitTripletGenerator) use it as the
base class type for dynamic polymorphism. Now we can also remove some
unneeded methods that are pure virtual in the base class.
Remove inheritance HitPairGenerator from HitPairGeneratorFromLayerPair

HitPairGeneratorFromLayerPair does not really benefit from the base
class. The users in practice use exactly this class (and not other
classes inheriting from HitPairGenerator), so they can safely be
modified to use the concrete type eliminating the need for dynamic
polymorphism. Also the HitPairGenerator::setSeedingLayers() can now be
removed as its asserting implmentation in CombinedHitPairGenerator.
Remove unnecessary methods from HitPairGenerator interface