Remove dummy variable declaration for context.Context from generated go code

Created by: moznion

Originally, this dummy declaration was needed because there was the possibility of the generated code doesn't use context.Context and then if it imported that package, go compiler complains that and makes an error.

ref: https://github.com/OpenAPITools/openapi-generator/blob/3ed1aa8e79687bed63dfa064de27317273080471/modules/swagger-codegen/src/main/resources/go/api.mustache#L30

However, now this dummy placement is no longer needed because the generated code always uses context.Context. So this pull request removes that dummy declaration.

cc. @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d

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