Draft: Some tidy-up for ShallowWaterSolver, CompressibleFlowSolver, and ADRSolver

Issue/feature addressed

Some tidy-up. Partially split from MR !1613 (merged).

Proposed solution

• Move SVVVarDiffCoeff from UnsteadySystem.h to UnsteadyViscousBurgers.cpp
• Delete unused function in NonlinearPeregrine.cpp

Implementation

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).