Remove outdated fork of `ConcurrentStatisticsImpl`

Prior to this change, OMERO was injecting a custom fork of the
`org.hibernate.stat.ConcurrentStatisticsImpl` class used by Hibernate.
It is not clear if the original issue leading to the need for this fork
remains, but regardless the fork is now out of date. It is missing a
number of methods in the Hibernate v4.3.6 version of the class.