chore: remove multiple cloud server flag (#12531)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - The "Add Selfhosted" and "Add Server" options are now displayed whenever the build configuration is native, without relying on feature flags.

- **Refactor**
  - Simplified conditional rendering for server addition buttons by replacing feature flag checks with build configuration checks.

- **Chores**
  - Removed the "Multiple Cloud Servers" feature flag and its related localization strings, streamlining feature management and UI labels.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/modules/feature-flag/constant.ts

@@ -209,15 +209,6 @@ export const AFFINE_FLAGS = {
     configurable: false,
     defaultState: isMobile,
   },
-  enable_multiple_cloud_servers: {
-    category: 'affine',
-    displayName:
-      'com.affine.settings.workspace.experimental-features.enable-multiple-cloud-servers.name',
-    description:
-      'com.affine.settings.workspace.experimental-features.enable-multiple-cloud-servers.description',
-    configurable: false,
-    defaultState: isDesktopEnvironment || BUILD_CONFIG.isIOS,
-  },
   enable_mobile_edgeless_editing: {
     category: 'affine',
     displayName: