Don't mention packed attribute in reference.md

src/doc/reference.md

@@ -1957,8 +1957,6 @@ macro scope.
   object file that this item's contents will be placed into.
 - `no_mangle` - on any item, do not apply the standard name mangling. Set the
   symbol for this item to its identifier.
-- `packed` - on structs or enums, eliminate any padding that would be used to
-  align fields.
 - `simd` - on certain tuple structs, derive the arithmetic operators, which
   lower to the target's SIMD instructions, if any; the `simd` feature gate
   is necessary to use this attribute.