Delete AuditEntry class
Delete AuditEntry class

This commit removes the AuditEntry class, along with all subclasses (i.e.
VersionAuditEntry and EditorialRemarkAuditEntry classes). This change doesn't
break any tests so there was no need for any more changes in the commit.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall