Remove calls to missing modules from gruntfile

When running the app, Grunt throws the following warnings:

>> Local Npm module "grunt-contrib-clean" not found. Is it installed?
>> Local Npm module "grunt-contrib-nodeunit" not found. Is it installed?

These modules don't appear to do anything except throw a warning so they can probably just be removed.

gruntfile.js

@@ -162,9 +162,6 @@ module.exports = grunt => {
 
 	});
 
-	grunt.loadNpmTasks('grunt-contrib-clean');
-	grunt.loadNpmTasks('grunt-contrib-nodeunit');
-
 	// Default task
 	grunt.registerTask( 'default', [ 'css', 'js' ] );