tamer: diagnose::report::Section::maybe_squash_into: Remove syslabel TODO

Previously, when adjacent duplicate spans were both resolved, if one failed,
the other certainly would, which would result in duplicate labels each
squash.  Elided spans do not have syslabels, and so this is no longer a
concern.

DEV-12151

tamer/src/diagnose/report.rs

@@ -310,9 +310,8 @@ impl<'s, 'd> Section<'d> {
     ) -> Option<Self> {
         match extend {
             Some(extend_sec) if self.span == extend_sec.span => {
-                // TODO: At the time of writing this will cause duplication of
-                //   system labels,
-                //     which is not desirable.
+                // Note that system labels shouldn't exist for elided spans
+                //   and so they should not be duplicated when squashing.
                 extend_sec.body.extend(
                     self.body
                         .into_iter()