Remove useless TRACK_DIAGNOSTIC calls.

There are two calls to `TRACK_DIAGNOSTIC` in
`DiagCtxtInner::emit_diagnostic` for cases where we don't emit anything
and the closure does nothing.

The only effect these calls have is to add a diagnostic to
`ImplicitCtxt::diagnostics`, which then gets added to
`QuerySideEffects::diagnostics`, which eventually gets handled by
`DepGraphData::emit_side_effects`, which calls `emit_diagnostic` on the
diagnostic, and again `emit_diagnostic` doesn't emit anything.

It's a big, complicate no-op. This commit removes it.

Author: nnethercote

compiler/rustc_errors/src/lib.rs

@@ -1294,20 +1294,16 @@ impl DiagCtxtInner {
                 return None;
             }
             Warning if !self.flags.can_emit_warnings => {
-                if diagnostic.has_future_breakage() {
-                    (*TRACK_DIAGNOSTIC)(diagnostic, &mut |_| {});
-                }
                 return None;
             }
             Allow | Expect(_) => {
-                (*TRACK_DIAGNOSTIC)(diagnostic, &mut |_| {});
                 return None;
             }
             _ => {}
         }
 
         let mut guaranteed = None;
-        (*TRACK_DIAGNOSTIC)(diagnostic, &mut |mut diagnostic| {
+        TRACK_DIAGNOSTIC(diagnostic, &mut |mut diagnostic| {
             if let Some(code) = diagnostic.code {
                 self.emitted_diagnostic_codes.insert(code);
             }