Remove noLMRExtension flag

This simplification patch removes the noLMRExtension flag. It was introduced in June
(see following link for that commit), but does not seem to be necessary anymore.
Link: https://github.com/official-stockfish/Stockfish/commit/e1f181ee643dcaa92c606b74b3abd23dede136cd
STC:
LLR: 2.94 (-2.94,2.94) <-2.50,0.50>
Total: 21200 W: 5369 L: 5228 D: 10603
Ptnml(0-2): 67, 2355, 5616, 2494, 68
https://tests.stockfishchess.org/tests/view/616c03d299b580bf37797dcb

LTC:
LLR: 2.94 (-2.94,2.94) <-2.50,0.50>
Total: 37536 W: 9387 L: 9278 D: 18871
Ptnml(0-2): 23, 3988, 10643, 4085, 29
https://tests.stockfishchess.org/tests/view/616c10f499b580bf37797ddd

closes https://github.com/official-stockfish/Stockfish/pull/3743

Bench: 4792969