Remove XA support for JCR sessions in ModeShape 5

ModeShape 3 and 4 had the capability of exposing a JcrSession instance as an XA resource. In other words, one could use 2PC explicitly with a JcrSession. The only reason why this worked and made sense what that the Infinispan cache itself was an XAResource, since many different caches could be part of the same cluster and therefore supporting distributed transactions was an option.

However, note that:
1. this was never really used by ModeShape - i.e. ModeShape always recommended that Infinispan would be configured as NON_XA
2. there were no specific use cases where this made sense for ModeShape's sessions.

Since ModeShape 5 will be using a shared storage for all repositories when running in a cluster together with the fact that exclusive locking is used whenever the central store is updated, this feature doesn't really make sense and it would also be complex to implement from scratch.

As such, the XA support for JcrSession should be removed.