Clean up E0198 explanation

src/librustc_error_codes/error_codes/E0198.md

@@ -1,17 +1,18 @@
+A negative implementation was marked as unsafe.
+
+Erroneous code example:
+
+```compile_fail
+struct Foo;
+
+unsafe impl !Clone for Foo { } // error!
+```
+
 A negative implementation is one that excludes a type from implementing a
 particular trait. Not being able to use a trait is always a safe operation,
 so negative implementations are always safe and never need to be marked as
 unsafe.
 
-```compile_fail
-#![feature(optin_builtin_traits)]
-
-struct Foo;
-
-// unsafe is unnecessary
-unsafe impl !Clone for Foo { }
-```
-
 This will compile:
 
 ```ignore (ignore auto_trait future compatibility warning)