Revert "Key backup: add `PUT /room_keys/version/{version}` to allow matrix clients to add signatures to an existing backup"

This reverts commit c9b38cbe53.

proposals/1219-storing-megolm-keys-serverside.md

@@ -177,42 +177,6 @@ Error codes:
 
 - `M_NOT_FOUND`: No backup version has been created.
 
-##### `PUT /room_keys/version/{version}`
-
-Update information about the given version, or the current version if `{version}`
-is omitted. Only `signatures` in `auth_data` can be updated.
-
-Body parameters:
-
-- `algorithm` (string): Optional. Must be the same as in the body parameters for `GET
- /room_keys/version`.
-- `auth_data` (object): Required. algorithm-dependent data.  For
-  `m.megolm_backup.v1.curve25519-aes-sha2`, see below for the definition of
-  this property.
-- `version` (string): Optional. The backup version. Must be the same as the query parameter or must be the current version.
-
-Example:
-
-```javascript
-{
-  "auth_data": {
-    "public_key": "abcdefg",
-    "signatures": {
-      "something": {
-        "ed25519:something": "hijklmnop"
-        "ed25519:anotherthing": "abcdef"
-      }
-    }
-  }
-}
-```
-
-On success, returns the empty JSON object.
-
-Error codes:
-
-- `M_NOT_FOUND`: No backup version has been created.
-
 #### Storing keys
 
 ##### `PUT /room_keys/keys/${roomId}/${sessionId}?version=$v`