Remove include Enumerable from ActiveLdap::Base

Since e7f6ab87dccd427d24f7430c826fca15734533ad there has been no ``#each`` method to power Enumerable and so it's now a redundant include.

This surfaces as a problem when using ``ActiveLdap::Base`` subclasses in the RSpec 3 [``match_array`` or ``contains_exactly``](https://www.relishapp.com/rspec/rspec-expectations/v/3-1/docs/built-in-matchers/contain-exactly-matcher) matchers as they try to map over the value if it is ``Enumerable`` (see: [Line 142 of ``with_matchers_cloned``](https://github.com/rspec/rspec-expectations/blob/master/lib/rspec/matchers/composable.rb#L117-149)).  ``ActiveLdap::Base`` instances claim they are ``Enumerable``, but have no ``.each`` method to power the ``map`` call.

Best option is just to remove it.