offload: Remove all the GC roots in case of multiple-output derivations.

* guix/scripts/offload.scm (remove-gc-root): Rename to...
  (remove-gc-roots): ... this.
  [builder]: Use 'scandir' and remove all the files starting with
  %GC-ROOT-FILE.
  (transfer-and-offload): Adjust to renaming; remove
  'false-if-exception' wraps.