keys.delete-keeper: Recovery-key labels are plain words.

This is checked when they're created (see `keys.new-recov').  Therefore,
it's not necessary to search recursively under `recov/' hoping to find
`current' links -- which is not especially reliable and somewhat
inefficient -- to enumerate all of the recovery keys.  Standard shell
globbing and checking for `current' links at the next level down is
sufficient and doesn't require messy parsing of find(1) output.