Remove references to gym-ignition-models

[traversaro]

The dependency was dropped upstream in ami-iit/adam#44, so I think we can also remove the reference here.

xref: robotology/gym-ignition-models#36

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[traversaro]

I do not bump the build number or re-render as this only changes the test dependencies, there is no need to upload a new package.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.