Remove unused headers

[jennuine]

🦟 Bug fix

Summary

Remove unused headers, which were causing the abi checker to fail (first detected in https://github.com/gazebosim/gz-gui/pull/486)

Checklist

• [x] 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)
• [x] 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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[methylDragon]

This PR needs https://github.com/gazebosim/gz-gui/pull/486 to pass I say we just force a merge :grimacing:, since we'll need a forced merge anyway to get the ABI checker to be happy.

Help! @scpeters ?

[scpeters]

@osrf-jenkins run tests please

[jennuine]

@scpeters https://github.com/gazebosim/gz-gui/pull/486 fixes INTEGRATION_ExamplesBuild_TEST but the abi checker is failing. This PR should fix abi but (from my understanding) not until after it's merged. I do not have privileges to override the merge