Make vim's JP-Mode OS-free

https://sites.google.com/site/fudist/Home/vim-nihongo-ban/vim-japanese/ime-control
ime_control.vim is now available on windows.
Revert "Make vim's JP-Mode OS-free"

This reverts commit d1b6db9876cf877c5c3e5cf46b3dc2a1e3063e96.