chore: remove IS_MESSAGING_CALENDAR_WEBHOOK_ENABLED flag (#22680)

Messaging/calendar webhook subscriptions are now always on; drop the
feature flag gate and its enum/public-flag registration.

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packages/twenty-server/src/engine/core-modules/feature-flag/constants/public-feature-flag.const.ts

@@ -28,14 +28,6 @@ export const PUBLIC_FEATURE_FLAGS: PublicFeatureFlag[] = [
         'Show the per-page hero illustration + video walkthrough modal on settings pages',
     },
   },
-  {
-    key: FeatureFlagKey.IS_MESSAGING_CALENDAR_WEBHOOK_ENABLED,
-    metadata: {
-      label: 'Messaging & Calendar Webhooks',
-      description:
-        'Sync Gmail, Google Calendar, and Microsoft 365 mail/calendar via provider push notifications instead of cron polling',
-    },
-  },
   ...(process.env.CLOUDFLARE_API_KEY
     ? [
         // {