Rollup merge of #120502 - clubby789:remove-ffi-returns-twice, r=compiler-errors

Remove `ffi_returns_twice` feature

The [tracking issue](https://github.com/rust-lang/rust/issues/58314) and [RFC](https://github.com/rust-lang/rfcs/pull/2633) have been closed for a couple of years.

There is also an attribute gate in R-A which should be removed if this lands.

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" {