SUNRPC: Remove unused declarations

Commit c7d7ec8f ("SUNRPC: Remove svc_shutdown_net()") removed
svc_close_net() implementation but left declaration in place. Remove
it.

Commit 1f11a034 ("SUNRPC new transport for the NFSv4.1 shared
back channel") removed svc_sock_create()/svc_sock_destroy() but not
the declarations.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>