Merge pull request #2526 from uhoreg/e2e_delete_backup

MSC2526: Add ability to delete key backups

proposals/2526-add-delete-backup.md

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+# MSC2526: 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.  If the specified version was
+previously deleted, the endpoint succeeds, returning an HTTP code of 200.  If
+the specified version never existed, the endpoint returns an HTTP code of 404
+with a Matrix `errcode` of `M_NOT_FOUND`.