NFS: Remove the left global variable nfs_callback_tcpport

Commit bbe0a3aa "NFS: make nfs_callback_tcpport per network context" has
make nfs_callback_tcpport per network, but left the global nfs_callback_tcpport,
remove it.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>