fs/common.mk: unbreak .xz compression when host-xz isn't built

fs/common.mk: unbreak .xz compression when host-xz isn't built

Closes #7160

Since f46f81de1315 (Don't build host-xz needlessly), host-xz is no longer
unconditionally built when xz fs compression is requested, so don't
explicitly refer to it.

Instead, rely on our check-host-xzcat logic to ensure xz is available and
set the path to ensure our host variant gets picked up if not available on
the build machine.

While we're at it, get rid of the now unused XZ variable.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>