Point /etc/resolv.conf to systemd-resolved stub resolver

The image generator script currently overrides the /etc/resolv.conf symbolic link to point directly to /run/systemd/resolve/resolv.conf. This is inconsistent, as it causes applications using glibc to use the stub resolver while applications that read /etc/resolv.conf (such as Go binaries) use the upstream resolver.

This commit removes the symlink override and lets the systemd-resolved package install the correct softlink to /run/systemd/resolve/stub-resolv.conf.