Remove unneeded special case to calculate size for has_many :through

All cases are properly handled in CollectionAssociation
for all subclasses of this association

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -13,21 +13,6 @@ def initialize(owner, reflection)
         @through_association = nil
       end
 
-      # Returns the size of the collection by executing a SELECT COUNT(*) query
-      # if the collection hasn't been loaded, and by calling collection.size if
-      # it has. If the collection will likely have a size greater than zero,
-      # and if fetching the collection will be needed afterwards, one less
-      # SELECT query will be generated by using #length instead.
-      def size
-        if has_cached_counter?
-          owner._read_attribute cached_counter_attribute_name(reflection)
-        elsif loaded?
-          target.size
-        else
-          super
-        end
-      end
-
       def concat(*records)
         unless owner.new_record?
           records.flatten.each do |record|