Remove unused MakeOptional

The only use was to work around a compiler warning in an ancient
compiler, which we no longer support.

src/optional.h

@@ -13,13 +13,6 @@
 template <typename T>
 using Optional = boost::optional<T>;
 
-//! Substitute for C++17 std::make_optional
-template <typename T>
-Optional<T> MakeOptional(bool condition, T&& value)
-{
-    return boost::make_optional(condition, std::forward<T>(value));
-}
-
 //! Substitute for C++17 std::nullopt
 static auto& nullopt = boost::none;