core: drop check for /etc/mtab

util-linux 2.27.1 now entirely stops looking at /etc/mtab, so we don't need to
verify /etc/mtab during early boot any more. Later on, tmpfiles.d/etc.conf will
fix /etc/mtab anyway, so there's not even a point in warning about it.

Drop test_mtab() and bump the util-linux dependency to >= 2.17.1.

Fixes #1495