maintenance: remove unused code from stream_vdi (#6054)

The code to extract vdis from geneva / zurich releases has been unused
for years

This comment was amusing:
```
                (* XXX: this is totally wrong: *)
```

ocaml/xapi/stream_vdi.ml

@@ -554,74 +554,3 @@ let recv_all refresh_session ifd (__context : Context.t) rpc session_id vsn
   let has_inline_checksums = vsn.Importexport.export_vsn > 0 in
   recv_all_vdi refresh_session ifd __context rpc session_id
     ~has_inline_checksums ~force
-
-(** Receive a set of VDIs split into chunks in a tar format created out of a Zurich/Geneva
-    exported VM. Each chunk has been independently compressed.*)
-let recv_all_zurich refresh_session ifd (__context : Context.t) rpc session_id
-    prefix_vdis =
-  TaskHelper.set_cancellable ~__context ;
-  TaskHelper.set_description ~__context "Importing Virtual Machine" ;
-  let progress = new_progress_record __context prefix_vdis in
-  (* The next header in the sequence *)
-  let hdr = ref None in
-  let next () =
-    hdr :=
-      try Some (Tar_unix.get_next_header ifd) with
-      | Tar.Header.End_of_stream ->
-          None
-      | e ->
-          raise e
-  in
-  next () ;
-  let recv_one ifd (__context : Context.t) (prefix, vdi_ref, _size) =
-    (* Open this VDI and stream in all the blocks. Return when hdr represents
-       a chunk which is not part of this VDI or the end of stream is reached. *)
-    with_open_vdi __context rpc session_id vdi_ref `RW [Unix.O_WRONLY] 0o644
-      (fun ofd _ ->
-        let rec stream_from (last_suffix : string) =
-          match !hdr with
-          | Some hdr ->
-              refresh_session () ;
-              let file_name = hdr.Tar.Header.file_name in
-              let length = hdr.Tar.Header.file_size in
-              if Astring.String.is_prefix ~affix:prefix file_name then (
-                let suffix =
-                  String.sub file_name (String.length prefix)
-                    (String.length file_name - String.length prefix)
-                in
-                if suffix <= last_suffix then (
-                  error
-                    "Expected VDI chunk suffix to have increased under \
-                     lexicograpic ordering; last = %s; this = %s"
-                    last_suffix suffix ;
-                  raise (Failure "Invalid XVA file")
-                ) ;
-                debug "Decompressing %Ld bytes from %s\n" length file_name ;
-                Gzip.Default.decompress ofd (fun zcat_in ->
-                    Tar_helpers.copy_n ifd zcat_in length
-                ) ;
-                Tar_helpers.skip ifd (Tar.Header.compute_zero_padding_length hdr) ;
-                (* XXX: this is totally wrong: *)
-                made_progress __context progress length ;
-                next () ;
-                stream_from suffix
-              )
-          | None ->
-              (* Since we don't count uncompressed bytes we aren't sure if we've
-                 		  really finished unfortunately. We can at least check to see if we
-                 	          were cancelled... *)
-              TaskHelper.exn_if_cancelling ~__context ;
-              ()
-        in
-        stream_from "" ; Unixext.fsync ofd
-    )
-  in
-  ( try for_each_vdi __context (recv_one ifd __context) prefix_vdis
-    with Unix.Unix_error (Unix.EIO, _, _) ->
-      raise
-        (Api_errors.Server_error (Api_errors.vdi_io_error, ["Device I/O error"]))
-  ) ;
-  if !hdr <> None then (
-    error "Failed to import XVA; some chunks were not processed." ;
-    raise (Failure "Some XVA data not processed")
-  )