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/i18n/src/i18n.gen.ts

@@ -5778,14 +5778,6 @@ export function useAFFiNEI18N(): {
       * `Let your words stand out. This also include the callout in the transcription block.`
       */
     ["com.affine.settings.workspace.experimental-features.enable-callout.description"](): string;
-    /**
-      * `Citation`
-      */
-    ["com.affine.settings.workspace.experimental-features.enable-citation.name"](): string;
-    /**
-      * `Enable citation feature.`
-      */
-    ["com.affine.settings.workspace.experimental-features.enable-citation.description"](): string;
     /**
       * `Link Preview Cache`
       */

packages/frontend/i18n/src/resources/en.json

@@ -1443,8 +1443,6 @@
   "com.affine.settings.workspace.experimental-features.enable-block-meta.description": "Once enabled, all blocks will have created time, updated time, created by and updated by.",
   "com.affine.settings.workspace.experimental-features.enable-callout.name": "Callout",
   "com.affine.settings.workspace.experimental-features.enable-callout.description": "Let your words stand out. This also include the callout in the transcription block.",
-  "com.affine.settings.workspace.experimental-features.enable-citation.name": "Citation",
-  "com.affine.settings.workspace.experimental-features.enable-citation.description": "Enable citation feature.",
   "com.affine.settings.workspace.experimental-features.enable-link-preview-cache.name": "Link Preview Cache",
   "com.affine.settings.workspace.experimental-features.enable-link-preview-cache.description": "Once enabled, the link preview will be cached and cached data will be used when the same link is fetched again. Otherwise, the link preview will be fetched every time.",
   "com.affine.settings.workspace.experimental-features.enable-embed-iframe-block.name": "Embed Iframe Block",