NFS: Remove unneeded forward declaration

I've built with NFSv4 enabled and disabled.  This forward
declaration does not seem to be required.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_xdr.h

@@ -14,9 +14,6 @@
 #define NFS_DEF_FILE_IO_SIZE	(4096U)
 #define NFS_MIN_FILE_IO_SIZE	(1024U)
 
-/* Forward declaration for NFS v3 */
-struct nfs4_secinfo_flavors;
-
 struct nfs4_string {
 	unsigned int len;
 	char *data;