Simplify Castle Extension

Remove depth condition in castle extension, also don't extend if
Singular Extension and Check Extansion fail to extend.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 42070 W: 9118 L: 9036 D: 23916
http://tests.stockfishchess.org/tests/view/5be899cc0ebc595e0ae32f07

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 78278 W: 12490 L: 12458 D: 53330
http://tests.stockfishchess.org/tests/view/5be8ac420ebc595e0ae33010

Bench: 3611041