Assorted trivial cleanups
Renaming doubleExtensions variable to multiExtensions, since now we have also triple extensions.
Some extra cleanups.
Recent tests used to measure the elo worth:
https://tests.stockfishchess.org/tests/view/659fd0c379aa8af82b96abc3
https://tests.stockfishchess.org/tests/view/65a8f3da79aa8af82b9751e3
https://tests.stockfishchess.org/tests/view/65b51824c865510db0272740
https://tests.stockfishchess.org/tests/view/65b58fbfc865510db0272f5b
closes https://github.com/official-stockfish/Stockfish/pull/5032
No functional change