xfs: Remove obsoleted m_inode_shrink from xfs_mount structure

Looks the old m_inode_shrink is obsoleted as we perform inodes reclaim per AG via
m_reclaim_workqueue, this patch remove it from the xfs_mount structure if so.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>