The BalancerDefragmentationPolicy and the BalancerChunkSelectionPolicy have both an abstract class and a single implementation of it.

Abstract classes are on balancer_defragmentation_policy.h and balancer_chunk_selection_policy.h
Implementations are on balancer_defragmentation_policy_impl.h and balancer_chunk_selection_policy_impl.h

We consider that there is no need to have an abstract class since there is only a single implementation and there is no plan to create any testing mock of them.

The aim of this ticket is to get rid of the abstract class files, remove the *impl.h suffix from the balancer_*_impl.h files and remove also the *Impl suffix of the BalancerDefragmentationPolicyImpl and BalancerChunkSelectionPolicyImpl classes.

Note: There is no need to do the same with `ClusterChunksResizePolicy` since it will go away on SERVER-66652