refactor, remove dead H5Dgather.hpp per-container overloads superseded by #276 dispatch

[steven-varga]

Context

After #276 rewired the write dispatch to use access_traits_t<T> × storage_representation_v<T>, the following overloads in H5Dgather.hpp are no longer called by any dispatch path:

Overload	Previously called by	Now
gather(list<T>), gather(deque<T>), gather(set<T>), gather(multiset<T>), gather(unordered_set<T>), gather(unordered_multiset<T>), gather(forward_list<T>)	H5Dwrite.hpp iterators branch	Dead — iterators branch now uses inline staging vector
gather(vector<string>) → char**	H5Dwrite.hpp pointers branch	Dead — pointers branch uses inline char* relay

The generic fallthrough gather(T, vector<E>&) remains live (called by the pointers/linear_value_dataset case for vector<NonTrivialPod>).

The h5::scatter<T> path is compiler-generated and unaffected.

Work

• Remove the 7 per-container gather overloads and the vector<string> char** overload from H5Dgather.hpp
• Update or remove test/H5Dgather.cpp tests that exercise these overloads directly (they call them as standalone utility API, so either migrate the tests to the dataset-level write path or delete them)
• Keep the generic gather(T, vector<E>&) fallthrough overload