Remove ffi_returns_twice feature

compiler/rustc_error_codes/src/error_codes/E0724.md

@@ -1,9 +1,12 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+
 `#[ffi_returns_twice]` was used on something other than a foreign function
 declaration.
 
 Erroneous code example:
 
-```compile_fail,E0724
+```compile_fail
 #![feature(ffi_returns_twice)]
 #![crate_type = "lib"]
 
@@ -15,7 +18,7 @@ pub fn foo() {}
 For example, we might correct the previous example by declaring
 the function inside of an `extern` block.
 
-```
+```compile_fail
 #![feature(ffi_returns_twice)]
 
 extern "C" {