Remove InferCtxt::err_count_on_creation.

It's no longer used meaningfully.

This also means `DiagCtxtHandle::err_count_excluding_lint_errs` can be
removed.

compiler/rustc_errors/src/lib.rs

@@ -925,18 +925,6 @@ impl<'a> DiagCtxtHandle<'a> {
         self.inner.borrow_mut().emit_stashed_diagnostics()
     }
 
-    /// This excludes lint errors, and delayed bugs.
-    #[inline]
-    pub fn err_count_excluding_lint_errs(&self) -> usize {
-        let inner = self.inner.borrow();
-        inner.err_guars.len()
-            + inner
-                .stashed_diagnostics
-                .values()
-                .filter(|(diag, guar)| guar.is_some() && diag.is_lint.is_none())
-                .count()
-    }
-
     /// This excludes delayed bugs.
     #[inline]
     pub fn err_count(&self) -> usize {