Remove obsolete second wakeup_paused

It was introduced in d5822af.
If I understand it correctly this was done to prevent calling select
without timeout while there were pending paused promises.

Since now we explicitely check for this condition and call select with
zero timeout (`should_block_waiting_for_io`). This extra `wakeup_paused`
seems to be obsolete.

CHANGES

@@ -89,6 +89,10 @@
 
   * Alias Lwt_result.map_err and Lwt_result.bind_lwt_err to Lwt_result.map_error and Lwt_result.bind_lwt_error for consistency with Stdlib. (#927, Antonin Décimo)
 
+====== Misc ======
+
+  * Resolve paused promises only once in main loop. This lets Lwt.pause behave identical to Lwt_unix.yield.(#917, Christopher Zimmermann)
+
 ===== 5.5.0 =====
 
 ====== Deprecations ======

src/unix/lwt_main.ml

@@ -26,8 +26,6 @@ let abandon_yielded_and_paused () =
 
 let run p =
   let rec run_loop () =
-    (* Fulfill paused promises now. *)
-    Lwt.wakeup_paused ();
     match Lwt.poll p with
     | Some x ->
       x
@@ -40,7 +38,7 @@ let run p =
         Lwt.paused_count () = 0 && Lwt_sequence.is_empty yielded in
       Lwt_engine.iter should_block_waiting_for_io;
 
-      (* Fulfill paused promises again. *)
+      (* Fulfill paused promises. *)
       Lwt.wakeup_paused ();
 
       (* Fulfill yield promises. *)