if i18n-ascii.txt is removed, reset the transliterate option since it is disabled and the admin can't do it

quite a corner case, but worth fixing anyway. If an admin creates the i18n-ascii.txt and then enables transliteration and then deletes the file they get into a situation where transliteration is enabled (checkbox checked) but the checkbox is disabled from interaction so they can't turn it off.

This small change automatically disables transliteration during a visit to admin/settings/pathauto if that file is missing.

In pathauto.inc the inclusion code is already pretty robust against that file disappearing so no change necessary there.