Remove Known problems section for vec_box (rust-lang#14252)

Remove Known problems section for `vec_box` since
[issue](rust-lang/rust-clippy#3530 (comment))
> 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

Author: Alexendoo

clippy_lints/src/types/mod.rs

@@ -61,10 +61,6 @@ declare_clippy_lint! {
     /// `Vec` already keeps its contents in a separate area on
     /// the heap. So if you `Box` its contents, you just add another level of indirection.
     ///
-    /// ### Known problems
-    /// Vec<Box<T: Sized>> makes sense if T is a large type (see [#3530](https://github.com/rust-lang/rust-clippy/issues/3530),
-    /// 1st comment).
-    ///
     /// ### Example
     /// ```no_run
     /// struct X {