Remove a superfluous subtrahend

The '- 1' subtrahend was introduced for guarding against null move
search at root, which would be nonsense. But this is actually already
guaranteed by the !PvNode condition. This followed from the discussion
in pull request 1609: #1609

No functional change

src/search.cpp

@@ -751,7 +751,7 @@ namespace {
         &&  ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
         && !excludedMove
         &&  pos.non_pawn_material(us)
-        && (ss->ply > thisThread->nmpMinPly || us != thisThread->nmpColor))
+        && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
     {
         assert(eval - beta >= 0);
 
@@ -780,7 +780,7 @@ namespace {
 
             // Do verification search at high depths, with null move pruning disabled
             // for us, until ply exceeds nmpMinPly.
-            thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4 - 1;
+            thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
             thisThread->nmpColor = us;
 
             Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);