whatnow: Remove undocumented printf-formatting of -prompt string.

whatnow(1)'s -prompt option would use the value as a printf(3) format
string, supplying `invo_name' as the sole value to be formatted.  This
was undocumented, and upset gcc's checking of format strings.  As no
requests to keep the ability appeared on nmh-workers in reply to
2017-08-09's email, treat -prompt's value as a simple string.