Add remove entity node (backport #629)

[mergify]

🎉 New feature

Summary

Add remove entity node (complementary to create)

Test it

Run gz sim with any entity and call remove launch as follows:

ros2 launch ros_gz_sim gz_remove_model.launch.py world:='world_name' entity_to_remove_name:='entity_name'

Test are also provided.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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 Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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This is an automatic backport of pull request #629 done by Mergify.