luci-app-ddns: fix some errors in pl/ddns.po

i18n-sync.sh reports some errors in pl/ddns.po:

- "File (Plik)" is duplicated
- "Notice (Spostrzeżenie)" is duplicated
- "Script (Skrypt)" is duplicated
- "-- custom -- (-- własne --)" is duplicated
- "Collecting data... (Trwa zbieranie danych...)" is duplicated
- "Version (Wersja)" is duplicated

This commit fixes these errors.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>