Update impermanence config

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/common/configs/system/nix-cleanup/cleanup.sh

@@ -18,13 +18,8 @@ if [[ -e /mnt/btrfs && -n $(mountpoint -q /mnt/btrfs) ]]; then
   exit 1
 fi
 
-if [[ -z "$DEVICE" ]]; then
-  echo "Error: DEVICE variable is not set."
-  exit 1
-fi
-
 mkdir -p /mnt/btrfs
-mount "/dev/mapper/$DEVICE" /mnt/btrfs
+mount "$DEVICE" /mnt/btrfs
 
 if [[ -e /mnt/btrfs/@.bak ]]; then
   if [[ -n "$(ls -A /mnt/btrfs/@.bak)" ]]; then