mark error code as removed

Author: RalfJung

compiler/rustc_error_codes/src/error_codes/E0691.md

@@ -1,9 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 A struct, enum, or union with the `repr(transparent)` representation hint
 contains a zero-sized field that requires non-trivial alignment.
 
 Erroneous code example:
 
-```compile_fail,E0691
+```ignore (error is no longer emitted)
 #![feature(repr_align)]
 
 #[repr(align(32))]