Get rid of clean::Function::header once #85833 is merged

[jyn514]

Originally posted by @jyn514 in #85833 (comment)

[yuvaldolev]

@rustbot claim

[jyn514]

Hi @yuvaldolev, #85833 was merged so you can start working on this :) feel free to ask for help on Zulip.

[yuvaldolev]

Thanks @jyn514!
I'll start working on it