NFS: Clean up nfsroot.c

Clean up: now that mount option parsing for nfsroot is handled
in fs/nfs/super.c, remove code in fs/nfs/nfsroot.c that is no
longer used.  This includes code that constructs the legacy
nfs_mount_data structure, and code that does a MNT call to the
server.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>