Remove unnecessary header file declaration and consistently use default keyword in StdRegions

Issue/feature addressed

The following changes have been made to StdRegions:

• Remove unnecessary header file declaration
• Add missing virtual keyword to virtual function declaration
• Consistently use default keyword for default constructor, copy constructor, and destructor
• Remove unused file StdLinSysKey.hpp
• Remove unused (commented) code.

Proposed solution

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).