Slight simplification in scale factor computation

Slight simplification in scale factor computation

[STC](http://tests.stockfishchess.org/tests/view/5b2614000ebc5902b8d17193)
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 17733 W: 3996 L: 3866 D: 9871

[LTC](http://tests.stockfishchess.org/tests/view/5b264d0f0ebc5902b8d17206)
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 55524 W: 9535 L: 9471 D: 36518

Use pawn count scaling also for opposite bishops endings with additional material, with a slope of 2 instead of 7. This simplifies slightly the code.

This PR is a functionally equivalent refactoring of the version which was submitted.

Four versions tried, 2 passed both STC and LTC. I picked the one which seemed more promising at LTC.

Slope 4 passed STC (-0.54 Elo), LTC not attempted
Slope 3 passed STC (+2.51 Elo), LTC (-0.44 Elo)
Slope 2 passed STC (+2.09 Elo), LTC (+0.04 Elo)
Slope 1 passed STC (+0.90 Elo), failed LTC (-3.40 Elo)

Bench: 4761613