maint: Do not xz-compress ISO images.

maint: Do not xz-compress ISO images.

The xz-compressed image is 23% smaller than the original ISO image (with
built-in zlib compression), but the extra decompression step is
unconventional and often a hindrance for users.  See discussion at
<https://lists.gnu.org/archive/html/guix-devel/2021-04/msg00497.html>.

* Makefile.am (release): Do not compress ISO images.
* doc/guix.texi (USB Stick and DVD Installation): Remove ".xz" suffix
from URL and file name.
(Copying to a USB Stick, Burning on a DVD): Remove introductory words,
@enumerate, and first item.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>