Refresh code from PR #2518, to fix review comments.

Author: marcalff

exporters/zipkin/include/opentelemetry/exporters/zipkin/zipkin_builder.h

@@ -23,9 +23,6 @@ class OPENTELEMETRY_EXPORT ZipkinBuilder
 public:
   static void Register(opentelemetry::sdk::configuration::Registry *registry);
 
-  ZipkinBuilder()           = default;
-  ~ZipkinBuilder() override = default;
-
   std::unique_ptr<opentelemetry::sdk::trace::SpanExporter> Build(
       const opentelemetry::sdk::configuration::ZipkinSpanExporterConfiguration *model)
       const override;

exporters/zipkin/src/zipkin_builder.cc

@@ -20,8 +20,6 @@ namespace exporter
 namespace zipkin
 {
 
-static ZipkinBuilder singleton;
-
 void ZipkinBuilder::Register(opentelemetry::sdk::configuration::Registry *registry)
 {
   auto builder = std::make_unique<ZipkinBuilder>();