journal-file-util: drop unused template argument for journal_file_open_reliably()

journal-file-util: drop unused template argument for journal_file_open_reliably()

I understand that the original motivation to introduce the template
argument here is to make journal_file_open() and _reliabrly() take the
same arguments. But, yeah, that's completely unused, not necessary to
complicate the code even the difference is not big.