bug#44205: [PATCH] Add new function seq-remove-item

[Juri Linkov <juri@linkov.net>]

> I found myself reaching for a version of `seq-remove' where I don't have
> to supply a lambda but can just give an item.  Ergo, the attached.
>
>     (seq-remove-item 2 '(1 2 3))  => (1 3)

There is no name 'item' used in function names of seq.el.  The term used
here is 'elt', so maybe 'seq-remove-elt' would be a better name.

> I find it a whole lot nicer than:
>
>     (seq-remove (lambda (a) (= a 2)) '(1 2 3))  => (1 3)
>
> Turns out it could already be used to simplify some code in tab-line.el,
> even if seq is arguably not yet very widely used in our sources.

The initial versions of tab-line.el used a more complex condition,
but now a simpler remove/remq could be used instead.