[front_end] Delete unused method getMessageCode.

This method was introduced in 2018 (see
https://dart-review.googlesource.com/c/78063) and never used.

Change-Id: I6a6a69640dc739012c9cbcd25c833358ab181d30
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448441
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
diff --git a/pkg/_fe_analyzer_shared/lib/src/messages/diagnostic_message.dart b/pkg/_fe_analyzer_shared/lib/src/messages/diagnostic_message.dart
index 53d03f7..6a3f3ea 100644
--- a/pkg/_fe_analyzer_shared/lib/src/messages/diagnostic_message.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/messages/diagnostic_message.dart
@@ -70,11 +70,6 @@
 }
 
 /// This method is subject to change.
-int getMessageCode(CfeDiagnosticMessage message) {
-  return message is FormattedMessage ? message.code.index : -1;
-}
-
-/// This method is subject to change.
 Map<String, dynamic>? getMessageArguments(CfeDiagnosticMessage message) {
   return message is FormattedMessage ? message.arguments : null;
 }