Remove obsolete usage of (ispell-insert-word)

Remove obsolete usage of (ispell-insert-word)

`ispell-insert-word' is a plain `insert' together with word filtering
through `translation-table-for-input' for character code unification.
This was useful in Emacs 22, but is not needed for Emacs 23 and above
since unification is now direct.

Since XEmacs does not have `translation-table-for-input' there is no
need at all to keep this old code, but use (insert) directly.