Remove eigen/include/ignition headers

[scpeters]

🦟 Bug fix

Split out from #622.

Summary

We forgot to remove the ignition headers from the eigen3 component, so do it now.

I expect the ABI checker job to fail for this PR since the ignition headers are still included on the base branch, but future ABI checks should be able to pass once gz-math8 is fixed.

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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