Remove PawnsOnBothFlanks

Remove PawnsOnBothFlanks

It looks like PawnsOnBothFlanks can be removed from initiative().
A barrage of tests seem to confirm that the adjustment to -110
does not gain elo to offset any potential loss by removing
PawnsOnBothFlanks.

STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 22014 W: 4760 L: 4639 D: 12615
http://tests.stockfishchess.org/tests/view/5b7f50cc0ebc5902bdbb3a3e

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 40561 W: 6667 L: 6577 D: 27317
http://tests.stockfishchess.org/tests/view/5b801f9f0ebc5902bdbb4467

The barrage of 0,4 tests on the -136 value are in my ps_tunetests branch.
http://tests.stockfishchess.org/tests/user/protonspring

Closes https://github.com/official-stockfish/Stockfish/pull/1751

Bench: 4413173

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How to continue from there?

The fact that endgames with all the pawns on only one flank are
drawish is a well-known chess idea, so it seems quite strange that
this can be removed so easily without losing Elo.

In the past there had been attempts to improve on PawnsOnBothFlanks
with similar concepts (for instance using the pawn span value), but
the tests were at best neutral. Maybe Stockfish is now mature enough
that these refined ideas would work to replace PawnsOnBothFlanks?