Remove ClearMaterialsCache call when a material is destroyed in ogre2 (backport #1110)

[mergify]

🦟 Bug fix

Summary

The material cache in Ogre2MeshFactory was introduced in #324 to clear material objects when a mesh is removed. Looking at the current state, this call should no longer be needed as we now have logic in Ogre2Material::Destroy function to destroy ogre internal material / texture objects (and also unregistering gz-rendering material objects) when they are not in use. So this PR removes the material cache and the the call to ClearMaterialCache function.

Note that we ran into an issue with the ClearMaterialsCache function. It clears materials by the name of the texture it contains. However, it's possible for multiple materials to use the same texture, and the current logic may incorrectly destroy the wrong material. Removing the call to ClearMaterialsCache resolves that issue.

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 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 #1110 done by [Mergify](https://mergify.com).