fix(docs): Remove unneeded postgres migration step

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2428>

changes/changelog.d/2124.doc

@@ -0,0 +1 @@
+Remove unnecessary postgres variable in Docker migration guide (#2124).

docs/administrator/migration.md

@@ -100,12 +100,6 @@ Follow this guide to migrate a mono-container installation to a multi-container
    cat .env.bak >> .env
    ```
 
-6. Update the database URL in your new `.env` file.
-
-   ```{code-block} sh
-   echo "DATABASE_URL=postgresql://funkwhale@postgres:5432/funkwhale" >> .env
-   ```
-
 Check the file and remove any duplicated settings after copying.
 
 ## Migrate your database