remove redundant declaration from generated code

It turns out that forward-declaring:
    struct foo;
is redundant when combined with:
    typedef struct foo foo;

Which is exactly what we were doing in the generated upb code. So, simplify it!

PiperOrigin-RevId: 456590031

upbc/protoc-gen-upb.cc

@@ -1331,9 +1331,6 @@ void WriteHeader(const FileLayout& layout, Output& output) {
       SortedExtensions(file);
 
   // Forward-declare types defined in this file.
-  for (auto message : this_file_messages) {
-    output("struct $0;\n", ToCIdent(message->full_name()));
-  }
   for (auto message : this_file_messages) {
     output("typedef struct $0 $0;\n", ToCIdent(message->full_name()));
   }