[GFS2] Remove duplicate sb reading code

[GFS2] Remove duplicate sb reading code

For some reason we had two different sets of code for reading in the
superblock. This removes one of them in favour of the other. Also we
don't need the temporary buffer for the sb since we already have one
in the gfs2 sb itself.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>