fix(rust-server): remove duplicate allow(unused_imports) directive

New versions of rust clippy reports errors with duplicate allow directives. This change fixes the duplicate directives in generated rust code.

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cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05) @jacob-pro (2022/10)

OpenAPITools / openapi-generator