rm cached attributes

The original patch that added this concept can be found [here](https://web.archive.org/web/20090601022739/http://dev.rubyonrails.org/ticket/9767). The current default behavior is to cache everything except serialized columns, unless the user specified otherwise. If anyone were to specify otherwise, many types would actually be completely broken. Still, the method is left in place with a deprecation warning in case anyone is actually still calling this method.
"Calling `cache_attributes` is no longer necessary. All attributes are cached."

The last sentence talking about downsides makes sense to Rails implementors, not app developers.