Statement has no effect in proxy_protocol.py

[pepoluan]

Frikken CodeQL does not realize it was a Protocol definition.

But let's just tidy them up, I think it got confused because the Protocol definition has a return statement there (which was added becase *I* was a bit confused about Protocol at that time 😅)

Tracking issue for:

• [ ] https://github.com/aio-libs/aiosmtpd/security/code-scanning/17
• [ ] https://github.com/aio-libs/aiosmtpd/security/code-scanning/18
• [ ] https://github.com/aio-libs/aiosmtpd/security/code-scanning/19