[Refactor] Remove unnecessary manual MPI_Reduce in hsolver dav code

[Cstandardlib]

Summary

• Remove unnecessary manual swap buffer and MPI_Reduce calls in diago_dav_subspace.cpp and diago_david.cpp
• reduce_pool template already has std::complex<float> and std::complex<double> instantiations with MPI_IN_PLACE, making manual buffer management redundant
• This simplifies the code by ~75 lines while maintaining the same functionality

Related Issues

Fixes #7313

Changes

source_hsolver/diago_dav_subspace.cpp

• Removed manual swap buffer allocation and syncmem_complex_op call
• Removed std::is_same<T, double>::value type check
• Removed manual MPI_Reduce calls for complex types
• Direct use of Parallel_Reduce::reduce_pool for both hcc and scc

source_hsolver/diago_david.cpp

• Same simplification as above for hcc reduce operation

Testing

• No functional changes; existing tests should pass
• The refactored code uses the same reduce_pool function that was already used for double type

[Copilot]

Pull request overview

This PR refactors the Davidson-based hsolver MPI reduction logic by removing manual temporary buffers and explicit MPI_Reduce calls, and instead consistently using Parallel_Reduce::reduce_pool for reducing hcc/scc contributions across MPI ranks.

Changes:

• Simplified MPI reduction paths in Davidson solvers by deleting manual swap buffer management and explicit MPI_Reduce calls.
• Unified reduction calls to Parallel_Reduce::reduce_pool(...) for both real and complex builds in DiagoDavid::cal_elem and Diago_DavSubspace::cal_elem.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
source/source_hsolver/diago_david.cpp	Replaces manual complex reduction logic with Parallel_Reduce::reduce_pool during hcc accumulation.
source/source_hsolver/diago_dav_subspace.cpp	Applies the same reduction simplification for hcc and scc accumulation.

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[mohanchen]

LGTM