DEV: Drop jQuery file uploader and old upload components (#15376)

This commit removes jQuery file uploader from Discourse,
completing the transition to Uppy. The image-uploader
and UploadMixin components are also removed in this commit
as they have already been replaced and are the only things
using jQuery file upload.

            .-'~~~`-.
          .'         `.
          |  R  I  P  |
          |   jquery  |
          |    file   |
          |   upload  |
          |           |
        \\| 2013-2021 |//
        -----------------

app/assets/javascripts/discourse/ember-cli-build.js

@@ -42,8 +42,6 @@ module.exports = function (defaults) {
   app.import(vendorJs + "bootbox.js");
   app.import(vendorJs + "bootstrap-modal.js");
   app.import(vendorJs + "jquery.ui.widget.js");
-  app.import(vendorJs + "jquery.fileupload.js");
-  app.import(vendorJs + "jquery.fileupload-process.js");
   app.import(vendorJs + "caret_position.js");
   app.import("node_modules/ember-source/dist/ember-template-compiler.js", {
     type: "test",