hooks/archiso: remove unneeded file system type from _mnt_dev

It was only used because util-linux did not support detecting EROFS and the mount command required -t erofs to mount EROFS.

util-linux 2.37 supports detecting EROFS; see https://github.com/util-linux/util-linux/commit/7b2a874e2ee306e9503210f8aa5eed7f53c06755. Now that the workaround is not needed anymore, it can be removed.