xfs: remove the unused xfs_log_iovec_t typedef

Source kernel commit: 3e5bdfe48e1f159de7ca3b23a6afa6c10f2a9ad2

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>