Remove the double auto-ref on arrays/strings as receivers

Part of #18469

[breaking-change]

A receiver will only ever get a single auto-reference. Previously arrays and strings would get two, e.g., [T] would be auto-ref'ed to &&[T]. This is usually apparent when a trait is implemented for `&[T]` and has a method takes self by reference. The usual solution is to implement the trait for `[T]` (the DST form).

r? @nikomatsakis (or anyone else, really)
This entire match is a noop, i.e. the `pick_step` method should just return `self.pick_autorefd_method(step)` (maybe in the `None` branch in the match above it?).