This is a non-functional simplification. Pawn_push and Up are redundant.
If we make up pawn_push, we can use it for all of the Up's and Down's.
In this version, I've also left the Up and Down constants so that there
is no worse readability.
STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 23878 W: 5202 L: 5085 D: 13591
http://tests.stockfishchess.org/tests/view/
5db5569a0ebc5902d6b14de4
Closes https://github.com/official-stockfish/Stockfish/pull/2378
No functional change
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