Revert "Fix invalid HTML"

This reverts commit d45f1dc31d85cd31dbf028bb2334c810b35ba107.

The change introduced a regression when removing a project from the
manage user page, because the "Remove" button is generated as an HTML
form (via print_project_user_list -> html_button() functions). Since
forms cannot be nested, the parent form's action (manage_user_proj_add.php)
gets triggered instead of the expected one (manage_user_proj_delete.php).

Fixes 0028106