xfs: Add xfs_attr_node_remove_name

This patch pulls a new helper function xfs_attr_node_remove_name out
of xfs_attr_node_remove_step.  This helps to modularize
xfs_attr_node_remove_step which will help make the delayed attribute
code easier to follow

Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>