ja: needless mnemonic label in Clear Applications Folder

The following buttons have needless mnemonic label in Japanese.

"Yes" and "No" in Clear Applications Folder dialog:
  - Select "Advanced" tab in Preferences
  - Click "Clear Folder" button to bring up Clear Applications Folder

"Yes" button in Japanese has "(Y)" and "No" button in Japanese has "(N)",
but these label strings are not needed because the buttons don't have
the function for accessibility.