Remove as useless micro-optimization in pawns generation (#1915)

Remove as useless micro-optimization in pawns generation (#1915)

The extra condition is used as a shortcut to skip the following 3 assignments:

```C++
        Bitboard b1 = shift<UpRight>(pawnsOn7) & enemies;
        Bitboard b2 = shift<UpLeft >(pawnsOn7) & enemies;
        Bitboard b3 = shift<Up     >(pawnsOn7) & emptySquares;
```

In case of EVASION with no target on 8th rank (the common case), we end up performing the 3 statements for nothing because b1 = b2 = b3 = 0.

But this is just a small micro-optimization and the condition is quite confusing, so just remove it and prefer a readable code instead.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 78020 W: 16978 L: 16967 D: 44075
http://tests.stockfishchess.org/tests/view/5c27b4fe0ebc5902ba135bb0

No functional change.