Further simplify move_importance()

Function move_importance() is already always
positive, so we don't need to add a constant
term to ensure it.

Becuase move_importance() is used to calculate
ratios of a linear combination (as explained in
previous patch), result is not affected. I have
also verified it directly.

No functional change.

src/timeman.cpp

@@ -44,7 +44,7 @@ namespace {
 
   double move_importance(int ply) {
 
-    return 1 / pow((1 + exp((ply - xshift) / xscale)), skewfactor) + 1e-3; // Ensure non-zero
+    return 1 / pow((1 + exp((ply - xshift) / xscale)), skewfactor);
   }