rustdoc: don't hyperlink square brackets in method signatures

[jsha]

For example, in https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.get, the method signature contains a [T]:

In that method signature, all four instances of [ or ] are colored and hyperlinked (to https://doc.rust-lang.org/nightly/std/primitive.slice.html).

These hyperlinks aren't very useful. For one thing, they present a very tiny click target, which is bad UX. For another thing, it's quite rare to be reading Rust documentation and needs to clarify that the [ and ] indicate a slice. We should remove these and save a little bit of page size.

[eggyal]

I've actually found those links to the slice docs quite useful, on a number of occasions.