Synapse supports unspecced DELETE /directory/list/appservice/<network ID>/<room ID> endpoint

[DMRobertson]

Like #13111, but for application services.

Defined here:

synapse/synapse/rest/client/directory.py

Lines 180 to 183 in fb9b361

	async def on_DELETE(
	self, request: SynapseRequest, network_id: str, room_id: str
	) -> Tuple[int, JsonDict]:
	return await self._edit(request, network_id, room_id, "private")

Spec is here and only defines a PUT.

Per #13111 (comment): we should

• check the number of requests matrix.org has seen to this endpoint
• check common appservices to see if they use it
• assuming it's unused, rip it out