LU-14521 flr: delete mirror without volatile file

Rather than opening a volatile file to delete a FLR mirror, this
patch delete sub objects on the specified mirror directly during
the mirror deletion handling.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I7a5e7488dbc820fdfa312218f363955a35752034
Reviewed-on: https://review.whamcloud.com/42116
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>