[Go] Fix duplication of "Api" in generated code when structPrefix is set

Created by: impl

In generated API requests/responses, this fixes a minor duplication of the word "Api", which is already part of the classname template parameter and doesn't need to be repeated when structPrefix is set.

For example, previously a generated name might be PetApiApiAddPetRequest, and with this change it is simplified to PetApiAddPetRequest.

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