Delete not used org.hippoecm.repository.jackrabbit.SessionImpl class

Hippo repository always and only uses the XASessionImpl extension of Jackrabbit XASessionImpl, never (anymore for a long time) the org.hippoecm.repository.jackrabbit.SessionImpl  extension of the Jackrabbit SessionImpl.

So our SessionImpl extension class actually is 'dead code', confusing to keep around.
And deleting it also removes the need for the SessionHelperImpl class which only is used to DRY encapsulate the shared functionalities between these two separate class hierarchies. Now that logic can simply be merged into the XASessionImpl extension class as its single remaining usage thereof.