Rollup merge of #99578 - steffahn:remove_redundant_bound, r=thomcc

Remove redundant lifetime bound from `impl Borrow for Cow`

The lifetime bound `B::Owned: 'a` is redundant and doesn't make a difference,
because `Cow<'a, B>` comes with an implicit `B: 'a`, and associated types
will outlive lifetimes outlived by the `Self` type (and all the trait's
generic parameters, of which there are none in this case), so the implicit `B: 'a`
implies `B::Owned: 'a` anyway.

The explicit lifetime bound here does however [end up in documentation](https://doc.rust-lang.org/std/borrow/enum.Cow.html#impl-Borrow%3CB%3E),
and that's confusing in my opinion, so let's remove it ^^

_(Documentation right now, compare to `AsRef`, too:)_
![Screenshot_20220722_014055](https://user-images.githubusercontent.com/3986214/180332665-424d0c05-afb3-40d8-a330-a57a2c9a494b.png)

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