SUNRPC: kernel PipeFS mount point creation routines removed

This patch removes static rpc_mnt variable and its creation and destruction
routines, because they are not used anymore.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>