Remove orderless-skip-highlighting

[minad]

This variable was added to support Selectrum, which has been deprecated now. Vertico and Corfu use a different technique to implement deferred highlighting for the visible candidates only.

There are no other users of this variable. The problem of deferred highlighting must be solved more generally by improving the completion styles API itself in minibuffer.el. It is therefore better to not provide this variable here.

[minad]

Oh, actually I found a user: https://github.com/jojojames/fussy/blob/0fbfcbace0729dba685da9b9a74566639b840ec3/fussy.el#L917-L920. Note that fussy is itself a completion style (or completion style framework), which can use Orderless, but it doesn't crucially depend on orderless-skip-highlighting.

Maybe we should then go via deprecation, if you want this.

EDIT: I added a deprecation just to be safe. Note that for special uses of Orderless (e.g. in fussy or as filter function somewhere else), orderless-filter can always be used directly if one does not want highlighting. Therefore we can remove this variable without a feature loss.

[minad]

@oantolin What do you think about this PR?

[oantolin]

The deprecation seems a little safer. Thanks!

[minad]

Thanks for merging!