xfs: kill the unused XFS_BB_FSB_OFFSET macro

Removing the macro, as this is no more needed in the code.
Tried to find the reference when it was last used - but the usage
for this seemed to have been dropped long time ago.

Signed-off-by: Amit Sahrawat <amit.sahrawat83@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>