fix(webtransport-websys): Allow poll_flush after poll_close

[oblique]

Description

This removes a debug_assert from poll_flush because it can be valid for the user to recheck the flush state. In that case the operation should be noop.

A real life code that triggers this assertition is this.

Fixes #5618

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates