Remove ignition related deprecations

[iche033]

🦟 Bug fix

Summary

Drop support for ignition - gz sim will no longer check for ignition related env variables or systems.

This breaks backward compatibility. Gazebo will not be able to load old state log files , or world files with ignition::gazebo systems, including ones on Fuel.

Update: Restored code to load ignition gazebo systems. There are currently still many models on fuel (outside of openrobotics org) that use the deprecated ign prefix so we will keep supporting them.

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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