replace 'disk_usage' with 'fs_info' from proxmox-sys

replace 'disk_usage' with 'fs_info' from proxmox-sys

Use the moved 'fs_info' helpers from the proxmox-sys crate (available
from there since proxmox-sys 0.3.0) as replacement for 'disk_usage'
in the workspace local tools crate and remove the latter as we do not
need it anymore.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
 [ T: squashed in removal of now unused import and reworded commit
  message to include version availability info, among other things ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>