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 |//
        -----------------

lib/tasks/javascript.rake

@@ -103,14 +103,6 @@ def dependencies
     }, {
       source: '@highlightjs/cdn-assets/.',
       destination: 'highlightjs'
-    }, {
-      source: 'blueimp-file-upload/js/jquery.fileupload.js',
-    }, {
-      source: 'blueimp-file-upload/js/jquery.iframe-transport.js',
-    }, {
-      source: 'blueimp-file-upload/js/jquery.fileupload-process.js',
-    }, {
-      source: 'blueimp-file-upload/js/vendor/jquery.ui.widget.js',
     }, {
       source: 'jquery/dist/jquery.js'
     }, {