Removed skip unmount option because it does not work.

Removed the following line, add later when it works again:
ubiquity ubiquity/partman-skip-unmount boolean true

preseed/lip.seed

@@ -3,4 +3,3 @@ d-i ubiquity/success_command string sh /isodevice/preseed/lip-success.sh
 d-i clock-setup/utc boolean false
 d-i time/zone string Europe/Berlin
 ubiquity ubiquity/use_nonfree boolean true
-ubiquity ubiquity/partman-skip-unmount boolean true