Refactoring: Remove the non-functional 'pdiagh' Pointer in hsolver

[Cstandardlib]

Describe the Code Quality Issue

Description

Original design use pointer pdiagh in base class HSolver to manage different iteration solvers. Now that we migrate to object instance of iterative solvers, which is clearer and reduces risk of memory insecurity, pdiagh will be redundant and has no practical effect on the functionality of the class.

Proposed Changes

Remove the 'pdiagh' pointer to clean up the code and prevent potential confusion for future maintainers.

Additional Context

No response

Task list for Issue attackers (only for developers)

• [ ] Identify the specific code file or section with the code quality issue.
• [ ] Investigate the issue and determine the root cause.
• [ ] Research best practices and potential solutions for the identified issue.
• [X] Refactor the code to improve code quality, following the suggested solution.
• [ ] Ensure the refactored code adheres to the project's coding standards.
• [ ] Test the refactored code to ensure it functions as expected.
• [ ] Update any relevant documentation, if necessary.
• [ ] Submit a pull request with the refactored code and a description of the changes made.