Remove obsolete Japanese translation for a while.

The translation is based on an early version of tutorial.md, thus most
of entries have been marked as fuzzy and actually they are incorrect.
Now tutorial.md is planed to be replaced with guide.md, so I'd suggest
removing translation files for a while.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #15959 : omasanori/rust/cleanup-ja, r=alexcrichton

The translation is based on an early version of tutorial.md, thus most
of entries have been marked as fuzzy and actually they are incorrect.
Now tutorial.md is planed to be replaced with guide.md, so I'd suggest
removing translation files for a while.

/cc @gifnksm