Remove dl.multiselect remnants

While working on #1309392: Remove _form_set_class(), I noticed that we somehow missed a couple of these in node-rtl.css and in the Seven's CSS files. The original issue is here: #732914: Improve the markup/CSS for content and user filter forms. Here are the grep results:

Problem code

$ grep -r dl.multiselect .
./modules/node/node-rtl.css:#node-admin-content dl.multiselect dd .form-item label {
./themes/seven/reset.css:dl.multiselect dd,
./themes/seven/reset.css:dl.multiselect dd .form-item,
./themes/seven/reset.css:dl.multiselect dd select,
./themes/seven/reset.css:dl.multiselect dt,
./themes/seven/reset.css:dl.multiselect .form-item,
./themes/seven/style-rtl.css:dl.multiselect,
./themes/seven/style-rtl.css:dl.multiselect dt,
./themes/seven/style-rtl.css:dl.multiselect dd {
./themes/seven/style.css:dl.multiselect,
./themes/seven/style.css:dl.multiselect dt,
./themes/seven/style.css:dl.multiselect dd {
./themes/seven/style.css:dl.multiselect select,
./themes/seven/style.css:dl.multiselect dd select {

How to fix

1. This code no longer exists in Drupal 7 or 8. It can be safely deleted, and backported to Drupal 7.
2. The #node-admin-buttons was part of this, so the entire node-rtl.css should be deleted in this patch.