Remove __storage.hpp

No longer used in favor of storage_for_completion_signatures (which is
reference aware).

Author: RobertLeahy

include/stdexec/__detail/__storage.hpp

@@ -56,7 +56,6 @@
 #include "__detail/__starts_on.hpp"
 #include "__detail/__stopped_as_error.hpp"
 #include "__detail/__stopped_as_optional.hpp"
-#include "__detail/__storage.hpp"
 #include "__detail/__storage_for_completion_signatures.hpp"
 #include "__detail/__submit.hpp"
 #include "__detail/__sync_wait.hpp"