async: minor refactorings (#15354)

lib/pure/asyncfutures.nim

@@ -157,33 +157,15 @@ proc checkFinished[T](future: Future[T]) =
       raise err
 
 proc call(callbacks: var CallbackList) =
-  when not defined(nimV2):
-    # strictly speaking a little code duplication here, but we strive
-    # to minimize regressions and I'm not sure I got the 'nimV2' logic
-    # right:
-    var current = callbacks
-    while true:
-      if not current.function.isNil:
-        callSoon(current.function)
-
-      if current.next.isNil:
-        break
-      else:
-        current = current.next[]
-  else:
-    var currentFunc = unown callbacks.function
-    var currentNext = unown callbacks.next
-
-    while true:
-      if not currentFunc.isNil:
-        callSoon(currentFunc)
-
-      if currentNext.isNil:
-        break
-      else:
-        currentFunc = currentNext.function
-        currentNext = unown currentNext.next
+  var current = callbacks
+  while true:
+    if not current.function.isNil:
+      callSoon(current.function)
 
+    if current.next.isNil:
+      break
+    else:
+      current = current.next[]
   # callback will be called only once, let GC collect them now
   callbacks.next = nil
   callbacks.function = nil