Remove unused method

The DOT::DOTElement.parent method has been commented out since its
initial addition in 2006 in 3e3f70e.

The Rubucop Layout/LeadingCommentSpace cop flagged it. Instead of
addressing that layout issue, this commit removes the method entirely.

Author: mhashizume

lib/puppet/external/dot.rb

@@ -151,11 +151,6 @@ def each_option
     def each_option_pair
       @options.each_pair{ |key, val| yield key, val }
     end
-
-    #def parent=( thing )
-    #    @parent.delete( self ) if defined?( @parent ) and @parent
-    #    @parent = thing
-    #end
   end
 
   # This is used when we build nodes that have shape=record