NFS: remove unused macros

MNT_fhs_status_sz/MNT_fhandle3_sz are never used after they were
introduced. So better to remove them.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>