feat(build): Make cleanup, license [sic] tasks invoke their prerequisites

Turns out they don't have any, so this commit just classifies
their gulp targets according to the established scheme.

scripts/gulpfiles/cleanup_tasks.js

@@ -81,5 +81,6 @@ function sortRequires() {
 };
 
 module.exports = {
+  // Manually-invokable targets, with prequisites where required.
   sortRequires: sortRequires
 };

scripts/gulpfiles/license_tasks.js

@@ -17,5 +17,6 @@ function checkLicenses() {
 };
 
 module.exports = {
+  // Manually-invokable targets, with prequisites where required.
   checkLicenses: checkLicenses
 };