rustdoc: Hide the "Show hidden undocumented items"

[tesuji]

I enabled in "Auto-hide trait implementations documentation" feature in Settings.
But the "Show hidden undocumented items" strings still displays and it is quite distracting to me:

It would be nice if it hide those strings completely by default.

Note: The screenshot was taken from https://doc.rust-lang.org/nightly/std/str/pattern/trait.Searcher.html#implementors.

[GuillaumeGomez]

It shouldn't appear in this case so this is a bug. Thanks for reporting it!

[GuillaumeGomez]

We don't show the implementors' items anymore so this is not relevant anymore.