Remove feature flag :webide_dark_theme

Needs to be shipped in 13.0

app/assets/javascripts/behaviors/markdown/render_mermaid.js

@@ -28,7 +28,6 @@ function importMermaidModule() {
 
       if (
         window.gon?.user_color_scheme === 'dark' &&
-        window.gon?.features?.webideDarkTheme &&
         // if on the Web IDE page
         document.querySelector('.ide')
       ) {

app/assets/javascripts/ide/components/ide.vue

@@ -57,7 +57,7 @@ export default {
       'editorTheme',
     ]),
     themeName() {
-      return this.glFeatures.webideDarkTheme && window.gon?.user_color_scheme;
+      return window.gon?.user_color_scheme;
     },
   },
   mounted() {

app/controllers/ide_controller.rb

@@ -6,10 +6,6 @@ class IdeController < ApplicationController
   include ClientsidePreviewCSP
   include StaticObjectExternalStorageCSP
 
-  before_action do
-    push_frontend_feature_flag(:webide_dark_theme)
-  end
-
   def index
     Gitlab::UsageDataCounters::WebIdeCounter.increment_views_count
   end