Revert "Fix potential unsafe initialization in the Graph class (#606)"

[scpeters]

🦟 Bug fix

This reverts commit 17585a9.

Summary

It appears that #606 and gazebosim/sdformat#1458 caused test failures in gz-sim (see osrf/buildfarm-tools#67 (comment)). I opened a test PR in gz-sim (gazebosim/gz-sim#2482) that builds against gz-math8 and sdformat15 with those changes reverted, and the tests were fixed:

https://build.osrfoundation.org/job/gz_sim-ci-pr_any-homebrew-amd64/637/

I don't know why this broke tests, but I propose reverting these changes for now until we can figure it out.

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)
• 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 Rebase-and-Merge.

[scpeters]

Cc @caguero

[scpeters]

gazebosim/sdformat#1459 needs to be merged before this