Add comment about removed error code.

Author: m-ou-se

compiler/rustc_error_codes/src/lib.rs

@@ -687,5 +687,6 @@ E0804: 0804,
 //  E0723, // unstable feature in `const` context
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
 //  E0744, // merged into E0728
+//  E0773, // Builtin macro defined more than once. This is no longer an error.
 //  E0776, // Removed; cmse_nonsecure_entry is now `C-cmse-nonsecure-entry`
 //  E0796, // unused error code. We use `static_mut_refs` lint instead.