remove description of known problems in match_same_arms that have already been resolved (rust-lang#13873)

The "known problems" pointed out in the `match_same_arms` lint seem to
have already been resolved.

changelog: none

Author: xFrednet

clippy_lints/src/matches/mod.rs

@@ -583,11 +583,6 @@ declare_clippy_lint! {
     /// are the same on purpose, you can factor them
     /// [using `|`](https://doc.rust-lang.org/book/patterns.html#multiple-patterns).
     ///
-    /// ### Known problems
-    /// False positive possible with order dependent `match`
-    /// (see issue
-    /// [#860](https://github.com/rust-lang/rust-clippy/issues/860)).
-    ///
     /// ### Example
     /// ```rust,ignore
     /// match foo {