Follow-up of request #13702

We should drop all remaining gulpfiles that are dedicated to extraction of gettext translated strings.
However three gulpfiles will remain at the end of this request:

1. The gulpfile at tuleap root that we need to launch all plugin build, convert Sass and concatenate legacy JS
2. The gulpfile in plugins/frs/www/js/angular/ . It converts Sass to CSS and should be adressed by request #13299
3. The gulpfile in plugins/tracker/www/scripts/ . It would be an ideal candidate because all it does is extract gettext, but following gerrit #15177, some strings are located in Vue files and therefore NOT extracted. As I don't really want to handle the part AngularJS part VueJS gettext extraction for now (ever ?), we should keep it as it is, otherwise every "make generate-po" invocation will drop all strings that are only in Vue files...