Simplify Stat Score bonus

This is a functional simplification of this statScore bonus.
There seems to be little risk of regression with this one.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 26829 W: 5892 L: 5781 D: 15156
http://tests.stockfishchess.org/tests/view/5c5086bb0ebc593af5d4db75

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 28232 W: 4684 L: 4575 D: 18973
http://tests.stockfishchess.org/tests/view/5c50d7690ebc593af5d4dec9

Closes #1979

Bench: 4001014

src/search.cpp

@@ -747,9 +747,7 @@ namespace {
     {
         if ((ss-1)->currentMove != MOVE_NULL)
         {
-            int p = (ss-1)->statScore;
-            int bonus = p > 0 ? (-p - 2500) / 512 :
-                        p < 0 ? (-p + 2500) / 512 : 0;
+            int bonus = -(ss-1)->statScore / 512;
 
             pureStaticEval = evaluate(pos);
             ss->staticEval = eval = pureStaticEval + bonus;