Drop generating "compat setter pointer" for optional set thrift fields

Summary:
Fire!

#build_rule_type[go_library,go_binary,go_unittest,go_test]

Reviewed By: echistyakov

Differential Revision: D68407032

fbshipit-source-id: dd22d5e655672244a9f98253842e97329c0679ed

third-party/thrift/src/thrift/compiler/generate/t_mstch_go_generator.cc

@@ -438,7 +438,7 @@ class mstch_go_field : public mstch_field {
     bool has_default_value = (field_->default_value() != nullptr);
     if (is_optional_() && has_default_value) {
       auto real_type = field_->type()->get_true_type();
-      bool is_container = (real_type->is_map() || real_type->is_set());
+      bool is_container = real_type->is_map();
       return is_container;
     }
     return is_pointer_();