Revert "Tweak reductions formula: 0.88 * depth + 0.12"

This patch reverts the recent commit called "Tweak reductions formula, etc."
The decisions for the revert decision were as follows:

1) The original commit called "Tweak reductions formula: 0.88 * depth + 0.12"
showed bad scaling at in a Very Long Time Control (VLTC) test:

VLTC (180+1.8):
LLR: -1.59 (-2.94,2.94) [0.00,5.00]
Total: 14968 W: 2247 L: 2257 D: 10464
http://tests.stockfishchess.org/tests/view/5b559ffa0ebc5902bdb84f36

2) So there was a suspicion that the original fast passing LTC test which lead
us to accept the patch may have been a statistical accident, so we organized
a match against the previous master at LTC to get an Elo estimate for the
patch:

LTC match:
ELO: -1.83 +-2.1 (95%) LOS: 4.3%
Total: 36018 W: 6018 L: 6208 D: 23792
http://tests.stockfishchess.org/tests/view/5b55f8110ebc5902bdb8526f

3) Based on these results, we ran a simplification test with [-3..1] bounds
for the revert at LTC:

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 41501 W: 7107 L: 7020 D: 27374
http://tests.stockfishchess.org/tests/view/5b5738670ebc5902bdb86932

4) So we revert.

Bench: 4491691