Remove asset_host_prefetch feature flag

This was added almost 2 years ago and has been enabled on GitLab.com
for a while now.

Removing this saves us a feature check on every HTML page rendered.

app/views/layouts/_head.html.haml

@@ -16,7 +16,7 @@
 %head{ prefix: "og: http://ogp.me/ns#" }
   %meta{ charset: "utf-8" }
 
-  - if Feature.enabled?('asset_host_prefetch') && ActionController::Base.asset_host
+  - if ActionController::Base.asset_host
     %link{ rel: 'dns-prefetch', href: ActionController::Base.asset_host }
     %link{ rel: 'preconnnect', href: ActionController::Base.asset_host, crossorigin: '' }
 

changelogs/unreleased/remove-asset-host-prefetch-feature-flag.yml

+---
+title: Prefetch DNS for asset host
+merge_request: 26868
+author:
+type: performance

spec/views/layouts/_head.html.haml_spec.rb

@@ -34,36 +34,23 @@ describe 'layouts/_head' do
     expect(rendered).to match(%{content="foo" http-equiv="refresh"})
   end
 
-  context 'when an asset_host is set and feature is activated in the config it will' do
+  context 'when an asset_host is set' do
     let(:asset_host) { 'http://assets' }
 
     before do
-      stub_feature_flags(asset_host_prefetch: true)
       allow(ActionController::Base).to receive(:asset_host).and_return(asset_host)
     end
 
-    it 'add a link dns-prefetch tag' do
+    it 'adds a link dns-prefetch tag' do
       render
-      expect(rendered).to match('<link href="http://assets" rel="dns-prefetch">')
-    end
-
-    it 'add a link preconnect tag' do
-      render
-      expect(rendered).to match('<link crossorigin="" href="http://assets" rel="preconnnect">')
-    end
-  end
 
-  context 'when an asset_host is set and feature is not activated in the config it will' do
-    let(:asset_host) { 'http://assets' }
-
-    before do
-      stub_feature_flags(asset_host_prefetch: false)
-      allow(ActionController::Base).to receive(:asset_host).and_return(asset_host)
+      expect(rendered).to match(%Q(<link href="#{asset_host}" rel="dns-prefetch">))
     end
 
-    it 'not add a link dns-prefetch tag' do
+    it 'adds a link preconnect tag' do
       render
-      expect(rendered).not_to match('<link href="http://assets" rel="dns-prefetch">')
+
+      expect(rendered).to match(%Q(<link crossorigin="" href="#{asset_host}" rel="preconnnect">))
     end
   end