Hi, here a patch simplyfing `message-simplify-subject' to make it
extensible (users can now add their own tailor-made one-arg function to
`message-simplify-subject-functions'), by removing all the explicit
checks and funcall, and just calling all the functions (implicit check
for `message-strip-list-identifiers' is moved to its body, and one for
`message-strip-subject-re' is removed based on previous patch from
<Bug#33200>).

This is my second patch, please tell-me if there’s a problem anywhere.
Not sure if I know how to make patch properly, nor to report bugs
properly.