rustdoc: make html checker a bit stricter

The old TODO comment stopped applying when
rust-lang#104129
stopped nesting the notable traits popover within the header.

src/tools/html-checker/main.rs

@@ -16,11 +16,6 @@ fn check_html_file(file: &Path) -> usize {
         "MISSING_ENDTAG_BEFORE",
         "INSERTING_TAG",
         "DISCARDING_UNEXPECTED",
-        // This error is caused by nesting the Notable Traits tooltip within an <h4> tag.
-        // The solution is to avoid doing that, but we need to have the <h4> tags for accessibility
-        // reasons, and we need the Notable Traits tooltip to help everyone understand the Iterator
-        // combinators
-        "TAG_NOT_ALLOWED_IN",
     ];
     let to_mute_s = to_mute.join(",");
     let mut command = Command::new("tidy");