i18n: align headers in kallithea.po files

Align the comment headers in all kallithea.po translation files:
- remove the description of the file: for new translations added via
Weblate, this is not updated and it'd contain 'Translations template for
Kallithea'. The name of the language is already present later in the file
in the metadata block.
- remove 'Automatically generated' lines present in some files
- add an empty line below the header block