Remove feature gates for Repository Languages

The repository languages feature got introduced through:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19480. This MR
included feature flags to guard against performance regressions, which
allowed GitLab to test it on gitlab.com before the customers would be
exposed.

.com worked out fine, as shown by our internal monitoring tools, which
prompts feature gate removal.

Given the docs weren't present yet, these were added too.

app/views/projects/show.html.haml

@@ -21,7 +21,6 @@
   %nav.project-stats{ class: [container_class, ("limit-container-width" unless fluid_layout)] }
     = render 'stat_anchor_list', anchors: @project.statistics_anchors(show_auto_devops_callout: show_auto_devops_callout)
     = render 'stat_anchor_list', anchors: @project.statistics_buttons(show_auto_devops_callout: show_auto_devops_callout)
-    - if Feature.enabled?(:repository_languages, @project.namespace.becomes(Namespace))
     = repository_languages_bar(@project.repository_languages)
 
 %div{ class: [container_class, ("limit-container-width" unless fluid_layout)] }

app/workers/detect_repository_languages_worker.rb

@@ -14,8 +14,6 @@ class DetectRepositoryLanguagesWorker
     user = User.find_by(id: user_id)
     return unless project && user
 
-    return if Feature.disabled?(:repository_languages, project.namespace)
-
     try_obtain_lease do
       ::Projects::DetectRepositoryLanguagesService.new(project, user).execute
     end

doc/user/project/repository/index.md

@@ -155,6 +155,16 @@ The repository graph displays visually the Git flow strategy used in that reposi
 
 Find it under your project's **Repository > Graph**.
 
+## Repository Languages
+
+For the default branch of each repository, GitLab will determine what programming languages
+were used and display this on the projects pages.
+
+![Repository Languages bar](img/repository_languages.png)
+
+Not all files are detected, among others; documentation, 
+vendored code, and most markup languages are excluded.
+
 ## Compare
 
 Select branches to compare and view the changes inline:

spec/services/projects/detect_repository_languages_service_spec.rb

@@ -5,10 +5,6 @@ describe Projects::DetectRepositoryLanguagesService, :clean_gitlab_redis_shared_
 
   subject { described_class.new(project, project.owner) }
 
-  before do
-    allow(Feature).to receive(:disabled?).and_return(false)
-  end
-
   describe '#execute' do
     context 'without previous detection' do
       it 'inserts new programming languages in the database' do