🐛 bug: remove prefork support from custom listeners

[efectn]

Description

We can't reuse old listeners for prefork. So it means, we're not able to use prefork with Listener(). I removed this part because of it's broken.
Fasthttp also doesn't support it.

Fixes #1254

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes

Commit formatting:

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