`no_proxy_helper` should not access `Container::value_type`

[see code here](https://github.com/boostorg/python/blob/master/include/boost/python/suite/indexing/detail/indexing_suite_detail.hpp#L487)

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.