Remove gulp makefile, since it's broken and unused

Looking at the history of the code, this `gulp` task was *accidentally* broken in the Gulp 4 upgrade in PR 10266 (all the way back in 2018).
Given that no one has either noticed and/or complained about this since, it's safe to assume that it's completely unused, hence why I suggest that we just remove this `gulp` task (rather than fixing it).

Author: Snuffleupagus

gulpfile.js

@@ -1848,17 +1848,6 @@ gulp.task("clean", function (done) {
   rimraf(BUILD_DIR, done);
 });
 
-gulp.task("makefile", function () {
-  let makefileContent = "help:\n\tgulp\n\n";
-  const targetsNames = [];
-  for (const i in target) {
-    makefileContent += i + ":\n\tgulp " + i + "\n\n";
-    targetsNames.push(i);
-  }
-  makefileContent += ".PHONY: " + targetsNames.join(" ") + "\n";
-  return createStringSource("Makefile", makefileContent).pipe(gulp.dest("."));
-});
-
 gulp.task("importl10n", function (done) {
   const locales = require("./external/importL10n/locales.js");