Remove the check for journal_callback_set() which was only needed
for ext3.

Also correct a couple of comments that still mentioned "ext3".

Change-Id: I0b4362eb1171051589f3772ddc253fa249019e6b
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/4678


Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Brian J. Murrell <brian.murrell@intel.com>