xfs: remove support for untagged lookups in xfs_icwalk*

With quotaoff not allowing disabling of accounting there is no need
for untagged lookups in this code, so remove the dead leftovers.

Repoted-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
[djwong: convert to for_each_perag_tag]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>