Fix: Resolve memory leak when using solvation model in relaxing geometry

[LKFEIYI]

Apologies for the previous PR #6832 (comment), which mixed bug fixes and refactoring. The refactoring was performed during the investigation of a memory leak issue #6794 (comment). Now the PR #6832 (comment) was divided into fix and refactor.

The previous memory leak was caused by induced_rho not being deleted in source/source_estate/module_pot/efield.cpp.

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[mohanchen]

LGTM, thanks

[mohanchen]

Thanks for your contribution!