refactor: start on removing FF for subscription tiers (#7596)

## Summary

Refactor: remove FF for subscription tier, remove legacy code for non
subscription tier logic.
 
## Review Focus

Preexisting cloud functionality impact.

<!-- If this PR fixes an issue, uncomment and update the line below -->
<!-- Fixes #ISSUE_NUMBER -->

## Screenshots (if applicable)

<!-- Add screenshots or video recording to help explain your changes -->

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7596-refactor-start-on-removing-FF-for-subscription-tiers-2cc6d73d3650816bac3aef893e4f37cd)
by [Unito](https://www.unito.io)

src/composables/useFeatureFlags.ts

@@ -13,7 +13,6 @@ export enum ServerFeatureFlag {
   MODEL_UPLOAD_BUTTON_ENABLED = 'model_upload_button_enabled',
   ASSET_UPDATE_OPTIONS_ENABLED = 'asset_update_options_enabled',
   PRIVATE_MODELS_ENABLED = 'private_models_enabled',
-  SUBSCRIPTION_TIERS_ENABLED = 'subscription_tiers_enabled',
   ONBOARDING_SURVEY_ENABLED = 'onboarding_survey_enabled'
 }
 
@@ -58,16 +57,6 @@ export function useFeatureFlags() {
         api.getServerFeature(ServerFeatureFlag.PRIVATE_MODELS_ENABLED, false)
       )
     },
-    get subscriptionTiersEnabled() {
-      // Check remote config first (from /api/features), fall back to websocket feature flags
-      return (
-        remoteConfig.value.subscription_tiers_enabled ??
-        api.getServerFeature(
-          ServerFeatureFlag.SUBSCRIPTION_TIERS_ENABLED,
-          true // Default to true (new design)
-        )
-      )
-    },
     get onboardingSurveyEnabled() {
       return (
         remoteConfig.value.onboarding_survey_enabled ??