Remove redundant code to convert restricted_levels to integers

The ApplicationSetting#restricted_visibility_levels= now takes care of
converting string formatted levels to integers.

app/controllers/admin/application_settings_controller.rb

@@ -52,16 +52,6 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
   end
 
   def application_setting_params
-    restricted_levels = params[:application_setting][:restricted_visibility_levels]
-
-    if restricted_levels.nil?
-      params[:application_setting][:restricted_visibility_levels] = []
-    else
-      restricted_levels.map! do |level|
-        level.to_i
-      end
-    end
-
     import_sources = params[:application_setting][:import_sources]
 
     if import_sources.nil?

spec/controllers/admin/application_settings_controller_spec.rb

@@ -19,14 +19,22 @@ describe Admin::ApplicationSettingsController do
       put :update, application_setting: { default_project_visibility: "20" }
 
       expect(response).to redirect_to(admin_application_settings_path)
-      expect(ApplicationSetting.current.default_project_visibility).to eq Gitlab::VisibilityLevel::PUBLIC
+      expect(ApplicationSetting.current.default_project_visibility).to eq(Gitlab::VisibilityLevel::PUBLIC)
     end
 
-    it 'falls back to default with default_project_visibility setting is omitted' do
+    it 'update the restricted levels for string values' do
+      put :update, application_setting: { restricted_visibility_levels: %w[10 20] }
+
+      expect(response).to redirect_to(admin_application_settings_path)
+      expect(ApplicationSetting.current.restricted_visibility_levels).to eq([10, 20])
+    end
+
+    it 'falls back to defaults when settings are omitted' do
       put :update, application_setting: {}
 
       expect(response).to redirect_to(admin_application_settings_path)
-      expect(ApplicationSetting.current.default_project_visibility).to eq Gitlab::VisibilityLevel::PRIVATE
+      expect(ApplicationSetting.current.default_project_visibility).to eq(Gitlab::VisibilityLevel::PRIVATE)
+      expect(ApplicationSetting.current.restricted_visibility_levels).to be_empty
     end
 
     context 'with valid repository_size_limit' do