Use single value for KingProtector.

After some recent big tuning session, the values for King Protector were
simplified to only be used on minor pieces. This patch tries to further
simplify by just using a single value, since current S(6,5) and S(5,6)
are close to each other. The value S(6,6) ended up passing, although
S(5,5) was also tried and failed STC.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 14261 W: 3288 L: 3151 D: 7822
http://tests.stockfishchess.org/tests/view/5b4ccdf50ebc5902bdb77f65

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 19606 W: 3396 L: 3273 D: 12937
http://tests.stockfishchess.org/tests/view/5b4ce4280ebc5902bdb7803b

Bench: 5448998