Avoid warnings by the Clang compiler

Clang gave a couple of warnings for unused parameters after the recnet commit "Use constexpr when makes sense".

No functional change.

src/types.h

@@ -333,7 +333,7 @@ inline Square& operator-=(Square &s, Direction d) { return s = s - d; }
 
 /// Only declared but not defined. We don't want to multiply two scores due to
 /// a very high risk of overflow. So user should explicitly convert to integer.
-Score operator*(Score s1, Score s2) = delete;
+Score operator*(Score, Score) = delete;
 
 /// Division of a Score must be handled separately for each term
 inline Score operator/(Score s, int i) {