`slice_or_array_pattern`: remove dead code.
`slice_or_array_pattern`: remove dead code.

After #62550, it is no longer possible for `slice`
to be other than `None | Some(Binding(..) | Wild)`.
In particular, `lower_pat_slice` may never generate
`Some(Array(..) | Slice(..))` and so there is nothing
to flatten into `slice`.

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust