GRMLBASE/98-clean-chroot: fix resolvconf symlink handling + get rid of resolvconf...

resolvconf no longer supports usage of /etc/resolvconf/run as such,
instead things are supposed to live in /run nowadays.  Quoting from
resolvconf's debian/changelog:

| resolvconf (1.80) unstable; urgency=medium
|
| [...]
|
|   [ Steve Langasek ]
|   * Eliminate all references to /etc/resolvconf/run.  This should all be done
|     directly in /run, there is no reason to support making any of this
|     configurable with a symlink since we already have a versioned dependency
|     on the version of initscripts that introduces the /run transition.

So /etc/resolv.conf being a symlink pointing to
/etc/resolvconf/run/resolv.conf no longer works, instead it needs to
point to /run/resolvconf/resolv.conf.

Given that our resolvconf code dates back to 2007 (see commit 1d1565d0e49),
while at it get rid of the workarounds, which don't seem to be needed
anymore.

Tested this change with builds against Debian/stretch (resolvconf 1.79),
Debian/buster (resolvconf 1.79) and Debian/bullseye (resolvconf 1.82).

Closes: https://github.com/grml/grml/issues/142