Delete note on Emacs 19 modes from comint.el

Delete note on Emacs 19 modes from comint.el

* lisp/comint.el: Delete note on how to convert pre-Emacs 20 modes to
use comint.el.  Emacs 20 was released in 1997, so it's safe to assume
that any relevant code has been updated in the last three decades.

(cherry picked from commit 4ece0e2e46b1eeb315b9ebcdcee09679319128f9)