Remove the `container_registry_api` feature flag

It has been enabled by default for a few milestones

config/feature_flags/development/container_registry_api.yml

----
-name: container_registry_api
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: true

lib/api/project_container_repositories.rb

@@ -8,7 +8,6 @@ module API
     REPOSITORY_ENDPOINT_REQUIREMENTS = API::NAMESPACE_OR_PROJECT_REQUIREMENTS.merge(
       tag_name: API::NO_SLASH_URL_PART_REGEX)
 
-    before { error!('404 Not Found', 404) unless Feature.enabled?(:container_registry_api, user_project, default_enabled: true) }
     before { authorize_read_container_images! }
 
     params do

spec/requests/api/group_container_repositories_spec.rb

@@ -25,7 +25,6 @@ RSpec.describe API::GroupContainerRepositories do
     group.add_reporter(reporter)
     group.add_guest(guest)
 
-    stub_feature_flags(container_registry_api: true)
     stub_container_registry_config(enabled: true)
 
     root_repository

spec/requests/api/project_container_repositories_spec.rb

@@ -31,7 +31,6 @@ RSpec.describe API::ProjectContainerRepositories do
     project.add_reporter(reporter)
     project.add_guest(guest)
 
-    stub_feature_flags(container_registry_api: true)
     stub_container_registry_config(enabled: true)
 
     root_repository