Remove offset in thread redistribution scheme.

Remove offset in thread redistribution scheme.

doesn't have a benefit.

passed STC (8 threads):
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 19574 W: 4028 L: 3904 D: 11642
http://tests.stockfishchess.org/tests/view/5b3e48950ebc5902b9fff080

passed LTC (8 threads):
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 21293 W: 3626 L: 3506 D: 14161
http://tests.stockfishchess.org/tests/view/5b3eefd60ebc5902b9fffa81

Closes https://github.com/official-stockfish/Stockfish/pull/1667

No functional change single threaded.