Remove the now redundant TT prefetch call from Position::do_move.

Tested for no regression and passed both
STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 48250 W: 9757 L: 9686 D: 28807

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 51412 W: 8887 L: 8816 D: 33709

No functional change.

Resolves #66