Document the removal of ENABLE_DESKTOP_NOTIFICATIONS

Author: dtdesign

docs/migration/wsc54/deprecations_removals.md

@@ -68,6 +68,7 @@ With version 5.5, we have deprecated certain components and removed several othe
 - `HTTP_SEND_X_FRAME_OPTIONS` ([WoltLab/WCF#4474](https://github.com/WoltLab/WCF/pull/4474))
 - `ELASTICSEARCH_ALLOW_LEADING_WILDCARD`
 - `WBB_MODULE_IGNORE_BOARDS` (The option will be always on with WoltLab Suite Forum 5.5 and will be removed with a future version.)
+- `ENABLE_DESKTOP_NOTIFICATIONS` ([WoltLab/WCF#4806](https://github.com/WoltLab/WCF/issues/4806))
 
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