Revert "packages: Enable threaded compression of source tarballs."

Threaded compression makes tarballs non-deterministic: the result depends on
the number of threads used for compressing.  See <https://bugs.gnu.org/31015>.

This reverts commit c8a3dea847.

guix/packages.scm

@@ -629,11 +629,6 @@ specifies modules in scope when evaluating SNIPPET."
               (apply invoke
                      (string-append #+tar "/bin/tar")
                      "cvf" #$output
-                     ;; The bootstrap xz does not support
-                     ;; threaded compression (introduced in
-                     ;; 5.2.0), but it ignores the extra flag.
-                     (string-append "--use-compress-program="
-                                    #+xz "/bin/xz --threads=0")
                      ;; avoid non-determinism in the archive
                      "--mtime=@0"
                      "--owner=root:0"