Remove todos_vue_application feature flag and purge all code

What does this MR do and why?

This removes the todos_vue_application feature flag which is already default_enabled: true in 17.8. We want do Remove all unused code after Haml version of To... (#509080 - closed) this milestone (%17.9).

With the flag, this MR also removes all old code that was on the flag false path. That's why this MR removes about 2k LOC

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual change. The new To-Do dashboard (Vue version) stays unchanged. The old dashboard (Haml version) was only reachable by manually setting the flag to false.

How to set up and validate locally

• 90% of the MR is just deleting obvious old code, in the first commit.
• The following commits remove individual helpers. For each I did a grep to make sure they weren' used anymore. But I'd apprechiate if you double check that Btw, there might be more helpers that we could remove, but when I hit the 2k LOC mark, I thought it's better to just open the MR. In case you spot opportunities for more code removal, I'll handle these as follow-ups.

app/helpers/todos_helper.rb

@@ -7,7+7,7 @@
 
   def todos_done_count
     @todos_done_count ||= current_user.todos_done_count
   end
 
   def todo_action_name(todo) # rubocop:disable Metrics/CyclomaticComplexity -- Will be removed/refactored as part of https://gitlab.com/gitlab-org/gitlab/-/issues/464069
     case todo.action
@@ -57,7+57,7 @@
     else
       # Note: Some todos (like for expired SSH keys) are neither related to a project nor a group.
       return unless todo.project.present?
 
       title = content_tag(:span, todo.project.name, class: 'project-name')
       namespace = content_tag(:span, "#{todo.project.namespace.human_name} / ", class: 'namespace-name')
 
@@ -67,7+67,7 @@
     end
   end
 
-  def todo_target_aria_label(todo)
-    target_type = if todo.for_design?
-                    _('Design')
-                  elsif todo.for_alert?
-                    _('Alert')
-                  elsif todo.member_access_requested?
-                    _('Group')
-                  elsif todo.for_issue_or_work_item?
-                    IntegrationsHelper.integration_issue_type(todo.target.issue_type)
-                  else
-                    IntegrationsHelper.integration_todo_target_type(todo.target_type)
-                  end
-
-    "#{target_type} #{todo_target_name(todo)}"
-  end
-
   def todo_target_path(todo)
     return unless todo.target.present?
 
     path_options = todo_target_path_options(todo)
 
     if todo.for_commit?
       project_commit_path(todo.project, todo.target, path_options)
     elsif todo.for_design?
       todos_design_path(todo, path_options)
     elsif todo.for_alert?
@@ -173,7+157,7 @@
       s_('Todos|Isn\'t an empty To-Do List beautiful?'),
       s_('Todos|Give yourself a pat on the back!'),
       s_('Todos|Nothing left to do. High five!'),
       s_('Todos|Henceforth, you shall be known as "To-Do Destroyer"')
     ]
   end