bug#63988: 30.0.50; Recent header line format changes cause spin/seg fault with format-mode-line

[Eli Zaretskii]

> Cc: 63988@debbugs.gnu.org, aaronjensen@gmail.com, monnier@iro.umontreal.ca
> Date: Thu, 15 Jun 2023 08:59:09 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> It sounds like the consensus here is that indeed this feature is not
> worth the complications, and so, unless I hear some good reasons not
> to do so, I intend to delete it in a week's time.

Time's up, so I've now reverted all the commits that dealt with this
feature, and I'm marking this bug done.