Move sar data colletion to sar plug-in

Historically the general plug-in collected the content of the
/var/log/sa directory while a separate sar plug-in handles the text
formatted summary files.

This makes little sense and adds to the bloat in the "general"
module; move the collection to the sar plug-in itself.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>