Remove attribute for display_warnings

[GuillaumeGomez]

From #73314 (comment).

[GuillaumeGomez]

I tried with:

#![doc(test(display_doctest_warnings))]

/// ```
/// let x = 12;
/// ```
pub fn foo() {}

No warning when running rustdoc --test foo.rs, however I got some when running rustdoc --test foo.rs -Z unstable-options --display-doctest-warnings. So I guess this issue doesn't exist. :)