process: replace validator

Replace the `validateFunction()` validator with the
`validateCallback()` validator to validate callbacks and keep
consistency.

lib/internal/process/task_queues.js

@@ -37,7 +37,6 @@ const FixedQueue = require('internal/fixed_queue');
 
 const {
   validateCallback,
-  validateFunction,
 } = require('internal/validators');
 
 const { AsyncResource } = require('async_hooks');
@@ -148,7 +147,7 @@ function runMicrotask() {
 const defaultMicrotaskResourceOpts = { requireManualDestroy: true };
 
 function queueMicrotask(callback) {
-  validateFunction(callback, 'callback');
+  validateCallback(callback);
 
   const asyncResource = new AsyncResource(
     'Microtask',