gitaly_client: Remove error handling for deprecated embedded errors
What does this MR do and why?
Gitaly has deprecated the use of embedded errors for the following RPCs:
- UserCreateBranch
- UserDeleteBranch
- UserCreateTag
- UserCherryPick
These embedded errors will be removed in %16.0 in favor of the new structured errors.
Rails has already been converted to correctly handle structured errors, which is proven by the fact that Gitaly hasn't been setting embedded errors for multiple releases already. Clean up the now-useless old error handling so that Gitaly can indeed remove the Protobuf fields. No change in behaviour is expected from this MR.
Part of #375572 (closed).
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