Clean up `blob_repository_vue_header_app` flag

.rubocop_todo/gitlab/feature_available_usage.yml

.rubocop_todo/layout/line_length.yml

.rubocop_todo/lint/unused_method_argument.yml

.rubocop_todo/rails/helper_instance_variable.yml

.rubocop_todo/rspec/context_wording.yml

.rubocop_todo/rspec/factory_bot/avoid_create.yml

.rubocop_todo/rspec/feature_category.yml

.rubocop_todo/rspec/named_subject.yml

.rubocop_todo/rspec/receive_messages.yml

.rubocop_todo/rspec/subject_declaration.yml

app/assets/javascripts/pages/projects/show/index.js

app/assets/javascripts/repository/components/header_area.vue

app/assets/javascripts/repository/index.js

app/controllers/projects/blob_controller.rb

app/controllers/projects/tree_controller.rb

app/controllers/projects_controller.rb

app/views/projects/_files.html.haml

app/views/projects/_readme.html.haml

app/views/projects/blob/_blob.html.haml

app/views/projects/buttons/_compare.html.haml