Remove a couple of outdated FIXME comments.

Remove a couple of outdated FIXME comments.

I had originally planned to implement a Compose-type key locally in
GTK PuTTY, as I did in Windows PuTTY. But in fact we've done this for
some time by delegating to the GTK IM system, which is a far better
idea anyway. So there's no point any more having the FIXME comment
that mentions Compose keys.

Also, there was a comment worrying about what I was going to do about
double-width characters in Pango, which is long since sorted out.