* lisp/international/titdic-cnv.el: Revert to utf-8 encoding

While it's true that using the iso-2022-jp encoding on the file does
allow Emacs to render the two strings differently, this only applies to
the source file.  The .elc files all use `utf-8-emacs` encoding anyway,
so that info is lost.  And the difference is even lost before we write
the .elc file because when Emacs byte-compiles that code the
byte-compiler considers those two strings as "equal" and emits only one
string in the byte-code (so the two branches return `eq` strings).

So, I think using `iso-2022-jp` is a bad idea here: it gives the
illusion that the the `charset` info exists, even it will be lost.
Eli discussed it with Handa-san a year ago, and they arrived at the
conclusion that the charset information is indeed no longer important.