Simplify the rust code generated by the generator (#1553)

Client: rs
diff --git a/compiler/cpp/src/thrift/generate/t_rs_generator.cc b/compiler/cpp/src/thrift/generate/t_rs_generator.cc
index df75d07..dce4288 100644
--- a/compiler/cpp/src/thrift/generate/t_rs_generator.cc
+++ b/compiler/cpp/src/thrift/generate/t_rs_generator.cc
@@ -2505,7 +2505,7 @@
   indent_up();
   f_gen_ << indent() << service_processor_name << " {" << endl;
   indent_up();
-  f_gen_ << indent() << "handler: handler," << endl;
+  f_gen_ << indent() << "handler," << endl;
   indent_down();
   f_gen_ << indent() << "}" << endl;
   indent_down();