no_proxy_helper should not access Container::value_type

[MichaelSuen-thePointer]

see code here

Some containers may not provide proper value_type member, and the user can still manually passing its element type as Key or Data template parameter to indexing_suite. So indexing_suite already know the container's value type, and does not need containers to have ::value_type. Its detailed implementation should then directly reading it from indexing_suite, not from Container::value_type again.