xfs: remove the xfs_btree_get_buf[ls] functions

Remove the xfs_btree_get_bufs and xfs_btree_get_bufl functions, since
they're pretty trivial oneliners.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>