Remove inheritance of HitTripletGenerator from CosmicHitTripletGenerator and CosmicHitTripletGeneratorFromLayerTriplet

These classes do not benefit from the base classes
(HitTripletGenerator, OrderedHitsGenerator). The classes are not
registered to any plugin factory, and hence can be constructed only
explicitly. The only user (SeedGeneratorForCosmics) use the type
CosmicHitTripletGenerator explicitly, so there is no need for dynamic
polymorphism from there either.
Remove inheritance of HitPairGenerator from CombinedHitPairGeneratorForPhotonConversion

CombinedHitPairGeneratorForPhotonConversion does not really benefit
from the base classes, and the user
(PhotonConversionTrajectorySeedProducerFromSingleLegAlgo) uses the
concrete type explicitly, so no need for dynamic polymorphism either.
In fact, now we can remove the non-implemented methods that are pure
virtual in the base class.
Remove inheritance of HitPairGenerator from HitPairGeneratorFromLayerPairForPhotonConversion

HitPairGeneratorFromLayerPairForPhotonConversion does not really
benefit from the base classes, and the only user
(CombinedHitPairGeneratorForPhotonConversion) uses the type
explicitly, so there is no need for dynamic polymorphism either. In
fact, now we can remove the non-implemented methods that are pure
virtual in the base class.
Remove inheritance of HitPairGenerator from CombinedHitQuadrupletGeneratorForPhotonConversion

CombinedHitQuadrupletGeneratorForPhotonConversion does not really benefit
from the base classes, and the sole user
(PhotonConversionTrajectorySeedProducerFromQuadrupletsAlgo) uses the
concrete type explicitly, so no need for dynamic polymorphism either.
In fact, now we can remove the non-implemented methods that are pure
virtual in the base class.
Remove inheritance of HitPairGenerator from HitQuadrupletGeneratorFromLayerPairForPhotonConversion

HitQuadrupletGeneratorFromLayerPairForPhotonConversion does not really
benefit from the base classes, and the only user
(CombinedHitQuadrupletGeneratorForPhotonConversion) uses the type
explicitly, so there is no need for dynamic polymorphism either. In
fact, now we can remove the non-implemented methods that are pure
virtual in the base class.
Remove inheritance of HitPairGenerator from CosmicHitPairGenerator

CosmicHitPairGenerator does not benefit from the base class, and the
users (SeedGeneratorForCRack and SeedGeneratorForCosmics) use the
concrete types, so there is no need for dynamic polymorphism either.
In fact, now we can remove some unneeded methods that are pure virtual
in the base class.
Remove inheritance of HitPairGenerator from CosmicHitPairGeneratorFromLayerPair

CosmicHitPairGeneratorFromLayerPair does not benefit from base
classes, and the user (CosmicHitPairGenerator) uses the type
explicitly, so there is no need for dynamic polymorphism either. In
fact, we can now remove some unnecessary methods that were pure
virtual in the base class.
Remove inheritance of MultiHitGenerator from MultiHitGeneratorFromPairAndLayers

MultiHitGeneratorFromPairAndLayers does not really benefit from the
base class, and the user (CombinedMultiHitGenerator) 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 of 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 of 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