Delete redundant lines in NS DoDiffusion

Issue/feature addressed

Minor tidy in Navier-Stokes DoDiffusion functions

Proposed solution

Variable converter no longer needs pressure to calculate temperature so pressure does not have to be stored

Implementation

Delete unnecessary lines

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).