Remove now unused error code

Author: oli-obk

src/librustc_mir/diagnostics.rs

@@ -597,21 +597,6 @@ See [RFC 911] for more details on the design of `const fn`s.
 [RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md
 "##,
 
-E0016: r##"
-Blocks in constants may only contain items (such as constant, function
-definition, etc...) and a tail expression. Erroneous code example:
-
-```compile_fail,E0016
-const FOO: i32 = { let x = 0; x }; // 'x' isn't an item!
-```
-
-To avoid it, you have to replace the non-item object:
-
-```
-const FOO: i32 = { const X : i32 = 0; X };
-```
-"##,
-
 E0017: r##"
 References in statics and constants may only refer to immutable values.
 Erroneous code example: