Utils: Remove variant.h

Since we are now requiring macOS 10.14 we can remove our local copy of
std::variant and use for macOS std::variant too.

Change-Id: I589d03b35fc56878b7392ffa7047a439e588fe43
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/sqlite/constraints.h

@@ -29,7 +29,8 @@
 
 #include <sqlitevalue.h>
 #include <utils/smallstring.h>
-#include <utils/variant.h>
+
+#include <variant>
 
 namespace Sqlite {
 
@@ -179,7 +180,7 @@ public:
     GeneratedAlwaysStorage storage = {};
 };
 
-using Constraint = Utils::variant<Unique,
+using Constraint = std::variant<Unique,
                                   PrimaryKey,
                                   ForeignKey,
                                   NotNull,