Remove !extension check #2045

Remove !extension check #2045

While looking at pruning using see_ge() (which is very valuable)
it became apparent that the !extension test is not adding any
value - simplify it away.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 56843 W: 12621 L: 12569 D: 31653
http://tests.stockfishchess.org/tests/view/5c8588cb0ebc5925cffe77f4

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 78622 W: 13223 L: 13195 D: 52204
http://tests.stockfishchess.org/tests/view/5c8611cc0ebc5925cffe7f86

Further work could be to optimize the remaining see_ge() test. The idea of less pruning at higher depths is valuable, but perhaps the test (-PawnValueEg * depth) can be improved.

Bench: 3188688