Apply suggestions from code review

Co-authored-by: Sergei Zimmerman <[email protected]>

Author: Radvendii

src/libexpr/include/nix/expr/nixexpr.hh

@@ -85,7 +85,7 @@ std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 
 using UpdateQueue = SmallTemporaryValueVector<conservativeStackReservation>;
 
-struct Exprs
+class Exprs
 {
     std::pmr::monotonic_buffer_resource buffer;
 public:
@@ -205,11 +205,6 @@ struct ExprString : Expr
         v.mkStringNoCopy(s);
     };
 
-    ~ExprString()
-    {
-        // String is allocated in an arena
-    }
-
     Value * maybeThunk(EvalState & state, Env & env) override;
     COMMON_METHODS
 };