sysvinit-core.postinst: remove code handling /etc/inittab with missing entries

In 2014, code was added to fix up /etc/inittab files on hurd which had /libexec/getty instead of /sbin/getty and which were missing the following line:

c:23:respawn:/sbin/getty 38400 console

Also since 2014 (commit 6af0110c) ./debian/share/inittab.gnu is installed on enduser systems which contains aforementioned fixes already.

This code removes these old codepaths together with the sysvinit/hurd-fix-inittab debconf template as new installations will already have a fixed /etc/inittab and old installations will have had the fixup code run already.

Removing this old codepath helps with DPKG_ROOT because when creating chrootless systems, uname -s output is potentially incorrect, for example when preparing a hurd chroot on linux.

Closes: #1063629