Admin users cannot remove appservice rooms from the directory

This issue has been migrated from [#5508](https://github.com/matrix-org/synapse/issues/5508).

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### Description

Administrator users cannot remove some rooms that were created by application services from the public directory.

### Steps to reproduce

- Install an appservice such as matrix-appservice-discord.
- Join a bridged room so that an entry is created in the public room directory.
- Open the room directory in Riot and select the current homeserver.
- Shift+click on the room and confirm the deletion.
- This results in "Error: This alias is reserved by an application service.".

Administrators should be allowed to remove any room from the directory.

### Version information

- **Homeserver**: federated
- **Version**: v1.0.0
- **Install method**: NixOS module and package.
- **Platform**: NixOS