async_hooks: remove unused useDomainTrampoline

Remove the useDomainTrampoline function and related domain_cb
variable that are no longer used after domain was ported to
AsyncLocalStorage.

Author: mcollina

lib/internal/async_hooks.js

@@ -111,25 +111,14 @@ const emitDestroyNative = emitHookFactory(destroy_symbol, 'emitDestroyNative');
 const emitPromiseResolveNative =
     emitHookFactory(promise_resolve_symbol, 'emitPromiseResolveNative');
 
-let domain_cb;
-function useDomainTrampoline(fn) {
-  domain_cb = fn;
-}
-
 function callbackTrampoline(asyncId, resource, cb, ...args) {
   const index = async_hook_fields[kStackLength] - 1;
   execution_async_resources[index] = resource;
 
   if (asyncId !== 0 && hasHooks(kBefore))
     emitBeforeNative(asyncId);
 
-  let result;
-  if (asyncId === 0 && typeof domain_cb === 'function') {
-    args.unshift(cb);
-    result = ReflectApply(domain_cb, this, args);
-  } else {
-    result = ReflectApply(cb, this, args);
-  }
+  const result = ReflectApply(cb, this, args);
 
   if (asyncId !== 0 && hasHooks(kAfter))
     emitAfterNative(asyncId);
@@ -623,7 +612,6 @@ module.exports = {
   pushAsyncContext,
   popAsyncContext,
   registerDestroyHook,
-  useDomainTrampoline,
   nativeHooks: {
     init: emitInitNative,
     before: emitBeforeNative,