Feature/cleanup texi

First, I removed the `*.sgml` files, because the sinfo format used is a custom format meant to be processed by sinfo.el, which uses the obsolete sgmls program. I spent some time trying to get this to work and failed. I don't see any advantage in keeping these and think we should switch to using .texi as the master file, or choose another format.

Second, I fixed the `mime-en.texi` file so that it can be processed by `texi2any`, the replacement for `makeinfo`.

Third, I corrected the encoding and title for `mime-ja.texi`. I could delete the broken crossreferences in this file as well, but if somebody who reads Japanese can correct them that would probably be better.