Revert include optional from experimental library

alib2gui/src/Converter.hpp

 #pragma once
 #include <tuple>
-
-#if __has_include(<optional>) && __cplusplus >= 201703L //FIXME has_include to allow build on gitlab-ci with alpine 3.7
-	#include <optional>
-#else
-	#include <experimental/optional>
-	namespace std {
-		template < class T >
-		using optional = experimental::optional < T >;
-
-		constexpr auto nullopt = std::experimental::nullopt;
-	}
-#endif
+#include <optional>
 #include <QString>
 #include <QImage>