OPENDJ-1689: Remove MatchingRule.comparator() and MatchingRuleImpl.comparator()

All matching rules now return a normalized representation which can be compared using the default byte string comparator. Also removed redundant equal length approximate matching rules which were only used for tests and weren't really needed.

Seeing the amount of boilerplate implementation in the server specific matching rules makes we wonder whether we should consider making the various AbstractMatchingRuleImpls public. Nicolas mentioned this when he was migrating the matching rules, and I think that I agree now I've had a closer look.