Remove select2

Once we are done migrating away from select2 as part of &8738 (closed), we should get rid of the dependency altogether.

Implementation plan

• Uninstall select2 package.
• Remove select2_overrides.scss.
• Search for the select2 keyword in the codebase to see what else can be removed.

Random reminders

• Delete app/views/shared/_new_project_item_select.html.haml
• Delete app/assets/javascripts/project_select.js
• Delete app/assets/javascripts/project_select_combo_button.js
• Delete the select2 stysheets.
• Cleanup e2e test helpers (qa/qa/page/component/dropdown.rb specifically).