Remove unused pop_span_label method

compiler/rustc_error_messages/src/lib.rs

@@ -455,10 +455,6 @@ impl MultiSpan {
         replacements_occurred
     }
 
-    pub fn pop_span_label(&mut self) -> Option<(Span, DiagMessage)> {
-        self.span_labels.pop()
-    }
-
     /// Returns the strings to highlight. We always ensure that there
     /// is an entry for each of the primary spans -- for each primary
     /// span `P`, if there is at least one label with span `P`, we return