Clean up default_to_import_tab experiment

app/controllers/import/bitbucket_server_controller.rb

app/controllers/import/fogbugz_controller.rb

app/controllers/import/github_controller.rb

app/controllers/import/gitlab_projects_controller.rb

app/controllers/import/manifest_controller.rb

app/views/projects/_import_project_pane.html.haml

config/feature_flags/experiment/default_to_import_tab.yml

ee/app/controllers/registrations/groups_controller.rb

ee/app/experiments/default_to_import_tab_experiment.rb

ee/app/helpers/registrations/groups_helper.rb

ee/app/views/registrations/groups/new.html.haml

ee/spec/controllers/registrations/groups_controller_spec.rb

ee/spec/experiments/default_to_import_tab_experiment_spec.rb

ee/spec/helpers/registrations/groups_helper_spec.rb

spec/controllers/import/bitbucket_server_controller_spec.rb

spec/controllers/import/fogbugz_controller_spec.rb

spec/controllers/import/github_controller_spec.rb

spec/controllers/import/manifest_controller_spec.rb

spec/requests/import/gitlab_projects_controller_spec.rb