proposal to add endpoint to delete backups

proposals/xxxx-add-delete-backup.md

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+# Add ability to delete key backups
+
+[MSC1219](https://github.com/matrix-org/matrix-doc/issues/1219) defined a
+mechanism for key backups.  However, it inadvertently omitted the endpoint to
+delete an entire key backup.  This proposal adds the endpoint.
+
+## Proposal
+
+An endpoint is added, `DELETE /room_keys/version/{version}`, that deletes a
+backup version.  Both the information about the key backup, as well as all keys
+associated with the backup should be deleted.  Like `POST
+/room_keys/version/{version}`, and unlike `GET /room_keys/version/{version}`,
+`{version}` cannot be empty, to ensure that the wrong backup is not
+accidentally deleted.