libgfs2: Move build_jindex() into fsck.gfs2
    
    fsck.gfs2 is the only remaining util that builds journals this way so
    move it out of libgfs2. The debug printfs can be removed because
    fsck.gfs2 doesn't enable libgfs2's cfg_debug.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>