Incorrect error note for empty implementations

[SiegeLord]

impl A for B;

gives this error:

test.rs:1:12: 1:13 error: obsolete syntax: empty implementation
test.rs:1 impl A for B;
                      ^
note: instead of `impl A;`, write `impl A {}`
error: aborting due to previous error

The note is talking about a different construct. It could use a more generic language to refer to trait implementations and non-trait implementations. I also question handling this via the obsolete syntax pathway, as it seems to leak historical details that are not too relevant for a Rust user.

[huonw]

triage: I've just removed the special handling in #11966, so it's now a normal syntax error to write impl A for B;. (Will be closed when that lands.)