Make sure potentially preexisting files are removed from $airootfs_dir before creating them with output redirection

mkarchiso creates two files in ${airootfs_dir} by using output redirection:

• printf '' > "${airootfs_dir}/etc/machine-id" in _cleanup_airootfs,
• printf '%s\n' "${iso_version}" > "${airootfs_dir}/version" in _make_version.

If any of these are existing symlinks, then the printf output would be written to the symlink target. It can be a big issue in case the symlink resolves to a path outside ${airootfs_dir}.

These files should be removed with rm -f before being are written to.