bug#61730: 30.0.50; Compiler warnings for delq and delete

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 61730@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>
> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Fri, 24 Feb 2023 14:43:34 +0100
> 
> Let's experiment: warning about for-effect calls to
> 
>   mapcar mapcan mapconcat
>   delq delete delete-dups delete-consecutive-dups
>   cl-delete cl-delete-if cl-delete-if-not cl-delete-duplicates
>   sort
> 
> results in 34 such calls found on master, most of them about `delq` and `delete`, but `delete-dups`, `cl-delete`, `mapconcat` and `sort` are also represented.
> 
> Some of these are no doubt safe, a few of them knowingly so, but it's definitely not obvious from a quick look at the code. It's poor style in any case.
> 
> Thus such a warning definitely falls on the beneficial side. Let's do it.

I'd hate to see Emacs's byte compiler becoming the Big Brother who
knows better.  Richard described a couple of situations where the
"problematic" usage is completely legitimate and safe; please try not
to emit the warning in those cases.