Core: Exclude callbacks & deferred modules in the slim build as well

So far, the slim build only excluded ajax & effects modules. As many web apps
right now rely on native Promises, often with a polyfill for legacy browsers,
deferred & callbacks modules are not that useful for sites that already exclude
ajax & effects modules.

This decreases the gzipped minified size of the slim module by 1760 bytes,
to 19706 bytes (below 20k!).

Closes gh-4553

build/tasks/build.js

@@ -306,7 +306,7 @@ module.exports = function( grunt ) {
 				// the official slim build
 				.reduce( ( acc, elem ) => acc.concat(
 					elem === "slim" ?
-						[ "-ajax", "-effects" ] :
+						[ "-ajax", "-callbacks", "-deferred", "-effects" ] :
 						[ elem ]
 				), [] )