archiso_pxe_common: remove resolv.conf before copy
Booting via PXE we want to keep our DNS configuration. So remove /etc/resolv.conf in new root before copying the current file.
Without this systemd-resolved fallback nameservers are used and we see an error message when the root ships a symbolic link to systemd-resolved's stub-resolv.conf:
cp: not writing through dangling symlink '/new_root/etc/resolv.conf'