Remove unused config.hh.in

[chapulina]

🦟 Bug fix

Summary

I believe that file isn't being used anywhere. I found no place generating it on CMake.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[methylDragon]

I kept it in just in case gz-cmake happens to call it in some utility function or something, but that was probably me being overly cautious.