Remove my own misleading comment

The `private` keyword does, in fact, make those instance methods
private. I think what I was trying to say is that the host class
(the class that VersionConcern is mixed into) will be able to use
these methods.  They *will* be private after being mixed in, but
they are not "api private", if that makes sense.

[ci skip]

lib/paper_trail/version_concern.rb

@@ -255,9 +255,6 @@ module PaperTrail
       @index ||= RecordHistory.new(sibling_versions, self.class).index(self)
     end
 
-    # TODO: The `private` method has no effect here. Remove it?
-    # AFAICT it is not possible to have private instance methods in a mixin,
-    # though private *class* methods are possible.
     private
 
     # @api private