Remove Known problems section for vec_box

[alex-semenyuk]

Remove Known problems section for vec_box since issue

Vec<Box<T: Sized>> makes sense if T is a large type

can be handled

The lint already checks that the type's size is lower than a threshold to avoid linting on large types.

changelog: none

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[alex-semenyuk]

r? clippy

[Alexendoo]

Thanks!