Tidy Set_Rhs_Magnitude function and remove unused function `v_NekSysInitialGuess`

Issue/feature addressed

• Remove unused protected function v_NekSysInitialGuess
• Remove deprecated version of Set_Rhs_Magnitude using NekVector<NekDouble> wrapper

Proposed solution

A summary of the proposed changes and how they address the issue/feature at hand. Whenever possible, describe alternatives your considered and decisions you made.

Implementation

A more detailed description of the changes. Please include any details necessary for reviewers to understand your work, and point out particular points would like them to pay particular attention to.

Tests

Suggested reviewers

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Notes

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Checklist

• [ ] Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).