Restore old aspiration window to 16

Tested directly at LTC because previous long
test series on this topic shows it is TC dependant.

Tested with no-regression mode because gets rid of
an ugly and ad-hoc rule.

Test at LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 67918 W: 10590 L: 10541 D: 46787

bench: 7926803

src/search.cpp

@@ -327,7 +327,7 @@ namespace {
             // Reset aspiration window starting size
             if (depth >= 5)
             {
-                delta = Value(depth > 23 ? 16 : 12);
+                delta = Value(16);
                 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
                 beta  = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
             }