gpg: Avoid extra translation strings.

* g10/keyedit.c (menu_expire): Use only one prompt.

The old code was anyway not fully i18n because it did not used
ngettext. Further we ran into const char*/char* conflicts on Windows
where we use a different gettext implementation.

FWIW: A better pattern in the case of a static and a malloced string
w/o error return on malloc failure would be:

const char *s;
char *s_buf;
s_buf = xtryasprintf ("%d foo", n);
if (!s_buf)
  s = "several foo";
else
  s = s_buf;
bar (s);
xfree (s_buf);