Remove unused autoconf checks
Removing the check for libio.h takes care of the warning mentioned in https://lists.gnu.org/archive/html/lilypond-devel/2021-08/msg00073.html .
Removing the check for libio.h takes care of the warning mentioned in https://lists.gnu.org/archive/html/lilypond-devel/2021-08/msg00073.html .