remove unused `#form_data?`

It’s used [only
once](https://github.com/rack/rack/blob/ab172af1b63f0d8e91ce579dd2907c43
b96cf82a/lib/rack/request.rb#L206) inside rack’s `#POST`.
However rails
[overrides](https://github.com/rails/rails/blob/e595d91ac2c07371b441f8b0
4781e7c03ac44135/actionpack/lib/action_dispatch/http/request.rb#L306)
rack’s `#POST`. So there is no calls for `#form_data?`.