remove unused `#form_data?`

It’s used [only once](https://github.com/rack/rack/blob/ab172af1b63f0d8e91ce579dd2907c43b96cf82a/lib/rack/request.rb#L206) inside rack’s `#POST`.
However rails [overrides](https://github.com/rails/rails/blob/e595d91ac2c07371b441f8b04781e7c03ac44135/actionpack/lib/action_dispatch/http/request.rb#L306) rack’s `#POST`. So there is no calls for `#form_data?`.