fix #4515: template setup: remove bogus LXC_NAME line from /etc/hosts

fix #4515: template setup: remove bogus LXC_NAME line from /etc/hosts

Templates not built directly by us, but redistributed from
images.linuxcontainer.org contain the following line in /etc/hosts:
`127.0.1.1       LXC_NAME`

While it is mostly cosmetic - LXC_NAME, contains an underscore and is
not a valid DNS name.

Simply drop it for newly created containers in template_fixup.

We ship Debian, Devuan ourselves, without this line - so no need for
modification there.

Currently the NixOS container-template (see [0]) also does not seem
to contain the line.

Noticed the issue with a fresh fedora container, where systemd-resolved
logs a warning upon boot.

[0] https://nixos.wiki/wiki/Proxmox_Linux_Container

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>