timers: remove domain enter and exit

With domains implemented over AsyncHooks, it's no longer
necessary to explicitly enter and exit the domain.

PR-URL: #17880
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>

lib/timers.js

@@ -256,15 +256,7 @@ function listOnTimeout() {
       continue;
     }
 
-    var domain = timer.domain;
-    if (domain) {
-      domain.enter();
-    }
-
     tryOnTimeout(timer, list);
-
-    if (domain)
-      domain.exit();
   }
 
   // If `L.peek(list)` returned nothing, the list was either empty or we have
@@ -688,31 +680,22 @@ var immediateQueue = new ImmediateList();
 function processImmediate() {
   var immediate = immediateQueue.head;
   var tail = immediateQueue.tail;
-  var domain;
 
   // Clear the linked list early in case new `setImmediate()` calls occur while
   // immediate callbacks are executed
   immediateQueue.head = immediateQueue.tail = null;
 
   while (immediate !== null) {
-    domain = immediate.domain;
-
     if (!immediate._onImmediate) {
       immediate = immediate._idleNext;
       continue;
     }
 
-    if (domain)
-      domain.enter();
-
     // Save next in case `clearImmediate(immediate)` is called from callback
     var next = immediate._idleNext;
 
     tryOnImmediate(immediate, tail);
 
-    if (domain)
-      domain.exit();
-
     // If `clearImmediate(immediate)` wasn't called from the callback, use the
     // `immediate`'s next item
     if (immediate._idleNext !== null)