Remove unused `Column#with_type`

Now that type casting has nothing to do with columns, our only usage of
this method is gone.

activerecord/lib/active_record/connection_adapters/column.rb

@@ -46,12 +46,6 @@ def human_name
         Base.human_attribute_name(@name)
       end
 
-      def with_type(type)
-        dup.tap do |clone|
-          clone.instance_variable_set('@cast_type', type)
-        end
-      end
-
       def ==(other)
         other.name == name &&
           other.default == default &&