fix old Japanese language code redirection

ja_JP has not been updated since March and the reference
to this language should be removed. It also exists as *ja*
on Transifex. Discussion is here: #10281 

@DeepDiver1975 @MorrisJobke 

This should be backported to stable7 and stable6 as well.

Later on, we will remove all obsolete ja_JP.php files.