rustdoc: per-type link colors in type declarations are gone

[QuietMisdreavus]

This is how the declaration of the Iterator trait looks in 1.27.1:

...but this is how it looks in 1.28.0 (current stable) as well as nightly:

The colors are still present down in the method declarations, so it's only a problem in the type declarations:

cc @GuillaumeGomez