refactor: make [Cohttp.Body.drain] private

Signed-off-by: Rudi Grinberg <[email protected]>

<!-- ps-id: 3cb40937-7500-4f1b-8e9a-a7d5cb282e4f -->

cohttp-async/src/body.mli

@@ -5,15 +5,14 @@ type t = [ Cohttp.Body.t | `Pipe of string Pipe.Reader.t ] [@@deriving sexp_of]
 
 include Cohttp.S.Body with type t := t
 
-val drain : t -> unit Deferred.t
-val is_empty : t -> [ `True | `False | `Unknown ]
 val to_string : t -> string Deferred.t
 val to_string_list : t -> string list Deferred.t
 val to_pipe : t -> string Pipe.Reader.t
 val of_pipe : string Pipe.Reader.t -> t
 val map : t -> f:(string -> string) -> t
 val as_pipe : t -> f:(string Pipe.Reader.t -> string Pipe.Reader.t) -> t
 val to_form : t -> (string * string list) list Deferred.t
+val is_empty : t -> [ `True | `False | `Unknown ]
 
 module Private : sig
   val write_body :