Remove unused ign_auto_headers.hh.in

[scpeters]

🦟 Bug fix

Removes an unused file

Summary

The ign_auto_headers.hh.in is provided by ign-cmake, but for some reason, this package has an old, unused copy in the source code. We can just remove it. Many other ign- packages have this as well, so it can be removed from there when convenient.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #305 (ccc2362) into ign-physics2 (c01cbad) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           ign-physics2     #305   +/-   ##
=============================================
  Coverage         82.84%   82.84%           
=============================================
  Files               108      108           
  Lines              4243     4243           
=============================================
  Hits               3515     3515           
  Misses              728      728           

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