Allow feature gate removal through the API

Features could be listed and added through the api, now also removed.
This was needed in the case of gitlab.com as the number of gates that
were ever used just grows and cleaning up is hard.

changelogs/unreleased/zj-feature-gate-remove-http-api.yml

+---
+title: Allow feature gates to be removed through the API
+merge_request:
+author:
+type: added

doc/api/features.md

@@ -86,3 +86,11 @@ Example response:
   ]
 }
 ```
+
+## Delete a feature
+
+Removes a feature gate. Response is equal when the gate exists, or doesn't.
+
+```
+DELETE /features/:name
+```

lib/api/features.rb

@@ -65,6 +65,13 @@ module API
 
         present feature, with: Entities::Feature, current_user: current_user
       end
+
+      desc 'Remove the gate value for the given feature'
+      delete ':name' do
+        Feature.get(params[:name]).remove
+
+        status 204
+      end
     end
   end
 end

spec/requests/api/features_spec.rb

 require 'spec_helper'
 
 describe API::Features do
-  let(:user)  { create(:user) }
-  let(:admin) { create(:admin) }
+  set(:user)  { create(:user) }
+  set(:admin) { create(:admin) }
 
   before do
     Flipper.unregister_groups
@@ -249,4 +249,43 @@ describe API::Features do
       end
     end
   end
+
+  describe 'DELETE /feature/:name' do
+    let(:feature_name) { 'my_feature' }
+
+    context 'when the user has no access' do
+      it 'returns a 401 for anonymous users' do
+        delete api("/features/#{feature_name}")
+
+        expect(response).to have_gitlab_http_status(401)
+      end
+
+      it 'returns a 403 for users' do
+        delete api("/features/#{feature_name}", user)
+
+        expect(response).to have_gitlab_http_status(403)
+      end
+    end
+
+    context 'when the user has access' do
+      it 'returns 204 when the value is not set' do
+        delete api("/features/#{feature_name}", admin)
+
+        expect(response).to have_gitlab_http_status(204)
+      end
+
+      context 'when the gate value was set' do
+        before do
+          Feature.get(feature_name).enable
+        end
+
+        it 'deletes an enabled feature' do
+          delete api("/features/#{feature_name}", admin)
+
+          expect(response).to have_gitlab_http_status(204)
+          expect(Feature.get(feature_name)).not_to be_enabled
+        end
+      end
+    end
+  end
 end