#575: remove explicit nullptr assignment

- Default "all zeros" sufficient; removes the
  apparent need/intent.

  Also, I'm not quite sure that an odd C++ couldn't assign the nullptr
  post-access and before the main method is called.
  C++17 has stronger guarantees.

  "The storage for objects with static storage duration (basic.stc.static) shall be zero-initialized (dcl.init) before any other initialization takes place. Zero-initialization and initialization with a constant expression are collectively called static initialization; ALL OTHER initialization is dynamic initialization..."

src/vt/trace/trace_containers.cc

@@ -4,9 +4,9 @@
 namespace vt { namespace trace {
 
 /*static*/ TraceContainerEventClassType*
-TraceContainers::event_type_container_{nullptr};
+TraceContainers::event_type_container_;
 
 /*static*/ TraceContainerEventType*
-TraceContainers::event_container_{nullptr};
+TraceContainers::event_container_;
 
 }} //end namespace vt::trace