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)

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.

I did not yet add it to NEWS or the manual; I will do that if people
agree that this is a good addition.

Comments?