Remove method duplicating built in functionality of attr_accessible

This method doesn't seem to have any reason to exist -- It provides
functionality that is built into attr_accessible (or with the
strong_parameters gem which is available now and replaces attr_accessible in
Rails 4).

The method was obtuse enough to require documentation nearly 3 times as
long as the actual method, and the code in the model and controller is
the same length without it.

I think it would be a good move to add strong_parameters to the Gemfile, and migrate to it in preparation for the release of Rails 4, but in the interim there's still no reason to add this much complexity to the code.