Remove not used code

[lukaszsamson]

Found by 1.19 typesystem. The build_params() call returns a non empty list so merge_all = merge_params ++ merge_private ++ merge_assigns is non empty as well.

     warning: comparison between distinct types found:

         merge_all != []

     given types:

         dynamic(non_empty_list(term(), term())) != empty_list()

     where "merge_all" was given the type:

         # type: dynamic(non_empty_list(term(), term()))
         # from: lib/phoenix/router/route.ex:178:15
         merge_all = merge_params ++ merge_private ++ merge_assigns

     While Elixir can compare across all types, you are comparing across types which are always disjoint, and the result is either always true or always false

     typing violation found at:
     │
 180 │     if merge_all != [] do
     │                  ~
     │
     └─ lib/phoenix/router/route.ex:180:18: Phoenix.Router.Route.build_prepare/1

[SteffenDE]

Thank you! 🙌🏻