Fix bizarre `if (!consttime_memeq(X, Y, N)!=0)' idioms.

Clang thinks the `!' is in the wrong place.  I think the `!=0' isn't
doing any work, so I've deleted it.

This stems from 5ad34db2ccbb "memcmp: Introduce and use
consttime_memeq",

  -    if (memcmp(m->nR,st->remoteN,NONCELEN)!=0) {
  +    if (!consttime_memeq(m->nR,st->remoteN,NONCELEN)!=0) {

at which time the !=0 was already redundant and became more confusing.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>