chore: remove citation feature flag (#12567)

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

- **New Features**
  - Footnote definitions and "Sources" headings are now always included in notes, without requiring a feature flag.
  - Enhanced footnote-related content with additional citation-style blocks such as bookmarks, embedded documents, and attachments.

- **Chores**
  - Removed the citation feature flag and its related configuration, logic, and translations from the application.

- **Documentation**
  - Updated localization files to remove entries related to the citation experimental feature.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/shared/src/services/feature-flag-service.ts

@@ -21,7 +21,6 @@ export interface BlockSuiteFlags {
   enable_table_virtual_scroll: boolean;
   enable_embed_doc_with_alias: boolean;
   enable_turbo_renderer: boolean;
-  enable_citation: boolean;
   enable_link_preview_cache: boolean;
   enable_dom_renderer: boolean;
 }
@@ -49,7 +48,6 @@ export class FeatureFlagService extends StoreExtension {
     enable_table_virtual_scroll: false,
     enable_embed_doc_with_alias: false,
     enable_turbo_renderer: false,
-    enable_citation: false,
     enable_link_preview_cache: false,
     enable_dom_renderer: false,
   });