Strip out commented-out translations after update

This is necessary because:
- Transifex runs 'msgfmt -c' on any uploaded .po file
- pybabel adds a spurious '#~ msgid ""' to the end of some files
- 'msgfmt -c' ignores that this is commented out, and treats it
  as a syntax error (clashing with the top file definition)

i2p2www/translations/de/LC_MESSAGES/priority.po

@@ -3182,9 +3182,3 @@ msgstr "Foren, Newsseiten, anderes"
 msgid "Very Old Stuff"
 msgstr "Sehr altes Material"
 
-#~ msgid ""
-#~ msgstr ""
-
-#~ msgid "Help channel"
-#~ msgstr "Hilfskanal"
-