block: remove target directory after unmounting

block: remove target directory after unmounting

This removes dangling directory fixing two issues:

1) Non autofs case
   Leaving directory in /mnt/ and switching to autofs would result in
   blockd failing to symlink().
2) autofs case
   Leaving directory in /var/run/blockd/ could result in apps trying to
   access it causing errors like:
   blockd: kernel is requesting a mount -> sda1
   blockd: failed to run block. add/sda1

Signed-off-by: Rafał Miłecki <[email protected]>