sane-backup-rsync-net: remove dead SANEBOX_PREPEND flags

i'm not actually sure how this is still working, with bunpen? but it *seems* to be

hosts/modules/services/rsync-net/sane-backup-rsync-net

@@ -28,9 +28,7 @@ for dir in "$@"; do
   # N.B.: manual flags instead of `-a -> -rlptgoD` because device files have a max path length which is too restricted
   # TODO: add `sane-vpn do unmetered --`, after fixing pasta/sane-vpn to preserve capabilities + not create a new user namespace unconditionally.
   # until then, don't run over cellular!
-  if SANEBOX_PREPEND="--sanebox-method landlock --sanebox-cap dac_read_search --sanebox-path $RN_ID" \
-    rsync --exclude="$RN_ID" -e "ssh -i $RN_ID" --mkpath -rlptgov --delete "$dir" "$remote_dir"; \
-  then
+  if rsync --exclude="$RN_ID" -e "ssh -i $RN_ID" --mkpath -rlptgov --delete "$dir" "$remote_dir"; then
     echo "$now" > "$dir"/zzz-rsync-net/last-completed
     rc=0$rc
   else