Remove unused environment variables from .github/workflow files

[Horiodino]

The GitHub Actions workflows publish-operator-opamp-bridge.yaml and publish-target-allocator.yaml included an unused step that read version information into environment variables (VERSION and VERSION_DATE).

https://github.com/open-telemetry/opentelemetry-operator/blob/main/.github/workflows/publish-operator-opamp-bridge.yaml#L35