remove the default parameter and updated comment

lib/arel/visitors/dot.rb

@@ -202,7 +202,7 @@ def visit_String o
 
       def visit_Hash o
         o.each_with_index do |pair, i|
-          edge("pair_#{i}")   { visit pair, a }
+          edge("pair_#{i}")   { visit pair }
         end
       end
 
@@ -216,7 +216,7 @@ def visit_edge o, method
         edge(method) { visit o.send(method) }
       end
 
-      def visit o = nil
+      def visit o
         if node = @seen[o.object_id]
           @edge_stack.last.to = node
           return

lib/arel/visitors/oracle.rb

@@ -89,7 +89,7 @@ def order_hacks o
         # Previous version with join and split broke ORDER BY clause
         # if it contained functions with several arguments (separated by ',').
         #
-        # orders   = o.orders.map { |x| visit x, a }.join(', ').split(',')
+        # orders   = o.orders.map { |x| visit x }.join(', ').split(',')
         orders   = o.orders.map do |x|
           string = visit x
           if string.include?(',')