fix(ci): Disable release mirroring in old repo

[pdobacz]

We don't need this (Ethereum is mirroring releases likely for some compatibility reasons which don't apply to us).

Monad spec test relesas will go always go only to https://github.com/monad-developers/execution-specs/releases

Greptile Summary

This PR stops mirroring fixture releases to the legacy repository. The main change is:

• Removes the cross-repository release step while keeping publication in monad-developers/execution-specs.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The canonical release still receives the generated fixture artifacts.
• No repository-local consumer depends on the removed mirror.

Important Files Changed

Filename	Overview
.github/workflows/release_fixture_feature.yaml	Removes the legacy mirror release step without changing fixture creation or canonical release publication.

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