Minor adjustment to avoid failing `make clean`

[eirikurj]

Purpose

I find the failing of make clean a little strange if the *.so have been deleted (e.g., by an earlier make clean) or are not found (have been moved by an external process). All other generated files are cleaned or fail silently if they do not exist.

This PR will make the behavior consistent when cleaning all generated files, including *.so files.

Expected time until merged

No rush.

Type of change

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [x] Refactoring (no functional changes, no API changes)
• [ ] Documentation update
• [x] Maintenance update
• [ ] Other (please describe)

Testing

Checklist

• [ ] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• [ ] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• [ ] I have run unit and regression tests which pass locally with my changes
• [ ] I have added new tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation

[codecov[bot]]

Codecov Report

Merging #22 (9edb26f) into main (3f2f060) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 9edb26f differs from pull request most recent head 0eb909d. Consider uploading reports for the commit 0eb909d to get more accurate results

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   49.97%   49.97%           
=======================================
  Files           5        5           
  Lines        1683     1683           
=======================================
  Hits          841      841           
  Misses        842      842           

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