Revert "remove_column does not take a type argument. [ci skip]. Closes rails#12864"

As of Rails 4.0 `remove_column` is no longer an alias for `remove_columns`.
The type is actually valid and used when issuing a rollback (new `change` method).

This reverts commit 9c9d494.

Author: senny

guides/source/migrations.md

@@ -179,7 +179,7 @@ generates
 ```ruby
 class RemovePartNumberFromProducts < ActiveRecord::Migration
   def change
-    remove_column :products, :part_number
+    remove_column :products, :part_number, :string
   end
 end
 ```