NFS: Remove unused nfs_rw_ops->rw_release() function

This was only ever set to nfs_writeback_release_common(), a function
which is completely empty.  Let's just drop this function pointer and
simplify the code a bit.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>