Remove unused DiagnosticBuilder::sub function

`Diagnostic::sub` is only ever used directly; it doesn't need to be
included in the builder.

compiler/rustc_errors/src/diagnostic.rs

@@ -582,6 +582,8 @@ pub fn styled_message(&self) -> &Vec<(String, Style)> {
 
     /// Convenience function for internal use, clients should use one of the
     /// public methods above.
+    ///
+    /// Used by `proc_macro_server` for implementing `server::Diagnostic`.
     pub fn sub(
         &mut self,
         level: Level,

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -157,19 +157,6 @@ pub fn buffer(self, buffered_diagnostics: &mut Vec<Diagnostic>) {
         buffered_diagnostics.extend(self.into_diagnostic().map(|(diag, _)| diag));
     }
 
-    /// Convenience function for internal use, clients should use one of the
-    /// span_* methods instead.
-    pub fn sub<S: Into<MultiSpan>>(
-        &mut self,
-        level: Level,
-        message: &str,
-        span: Option<S>,
-    ) -> &mut Self {
-        let span = span.map(|s| s.into()).unwrap_or_else(MultiSpan::new);
-        self.0.diagnostic.sub(level, message, span, None);
-        self
-    }
-
     /// Delay emission of this diagnostic as a bug.
     ///
     /// This can be useful in contexts where an error indicates a bug but