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 -->

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

@@ -106,16 +106,6 @@ export const AFFINE_FLAGS = {
     configurable: isCanaryBuild,
     defaultState: isCanaryBuild,
   },
-  enable_citation: {
-    category: 'blocksuite',
-    bsFlag: 'enable_citation',
-    displayName:
-      'com.affine.settings.workspace.experimental-features.enable-citation.name',
-    description:
-      'com.affine.settings.workspace.experimental-features.enable-citation.description',
-    configurable: isCanaryBuild,
-    defaultState: isCanaryBuild,
-  },
   enable_link_preview_cache: {
     category: 'blocksuite',
     bsFlag: 'enable_link_preview_cache',