sunrpc: drop "select NETVM"

The Kconfig entry for SUNRPC_SWAP selects NETVM. That select statement
was added in commit a564b8f0 ("nfs:
enable swap on NFS"). But there's no Kconfig symbol NETVM. It apparently
was only in used in development versions of the swap over nfs
functionality but never entered mainline. Anyhow, it is a nop and can
safely be dropped.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>