Simplify endgame factor for opposite colored bishops

Stockfish is continually improving. Patches that gain elo in the past may
no longer be needed as stockfish improved elsewhere. This patch removes
passed pawns count dependence in opposite colored bishops scale factor.
We used the mean of passed count pawns (~1.4) to compensate, and changed
the base value from 16 to 22.

Passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 57879 W: 12657 L: 12607 D: 32615
http://tests.stockfishchess.org/tests/view/5dd1644f42928ff08153dc1e

Passed LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 121648 W: 19622 L: 19659 D: 82367
http://tests.stockfishchess.org/tests/view/5dd24572ccb823d41d4b47bb

Closes #2419

Bench: 5067864

src/evaluate.cpp

@@ -748,7 +748,7 @@ namespace {
     {
         if (   pos.opposite_bishops()
             && pos.non_pawn_material() == 2 * BishopValueMg)
-            sf = 16 + 4 * pe->passed_count();
+            sf = 22 ;
         else
             sf = std::min(sf, 36 + (pos.opposite_bishops() ? 2 : 7) * pos.count<PAWN>(strongSide));