Remove dead attr_reader: `GRPC::SemianError::details`

[shopify-codedb[bot]]

This attr_reader appears to be unused and could be removed.

Before approving this pull-request, please double-check that it is indeed unused.

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If this code is actually used, please add a comment explaining why and close this pull-request.

You can find more unused code in your project at: https://code.shopify.io/projects/shopify/semian/code_removals/spoom

Note: closing this pull-request will mark the code as ignored and exclude it from future dead code detection.

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This pull request was created by @conrad-mo.