Remove redundant check
We still check for `rustc_on_unimplemented` on implementations, but this functionality was removed in https://github.com/rust-lang/rust/pull/139091, since then it always returns `Ok` when called with a non-trait defid.
b4c8b0c3f0/compiler/rustc_trait_selection/src/error_reporting/traits/on_unimplemented.rs (L557-L564)