SUNRPC: rpc_call_null_helper() already sets RPC_TASK_NULLCREDS

Clean up.

Commit a52458b4 ("NFS/NFSD/SUNRPC: replace generic creds with
'struct cred'.") made rpc_call_null_helper() set RPC_TASK_NULLCREDS
unconditionally. Therefore there's no need for
rpc_call_null_helper()'s call sites to set RPC_TASK_NULLCREDS.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>