Remove http_message_signatures feature flag (#36610)

app/javascript/mastodon/utils/environment.ts

@@ -12,7 +12,7 @@ export function isProduction() {
   else return import.meta.env.PROD;
 }
 
-export type Features = 'modern_emojis' | 'fasp' | 'http_message_signatures';
+export type Features = 'modern_emojis' | 'fasp';
 
 export function isFeatureEnabled(feature: Features) {
   return initialState?.features.includes(feature) ?? false;