Make mojo_system static and mojo_system_impl a component, never use both

mojo/system/simple_dispatcher.h

Index: mojo/system/simple_dispatcher.h
diff --git a/mojo/system/simple_dispatcher.h b/mojo/system/simple_dispatcher.h
index d2934faa997c25997e53c569ac47e053f1675749..15a64d58d28bc13e413adf9f2b0b5361587bd42e 100644
--- a/mojo/system/simple_dispatcher.h
+++ b/mojo/system/simple_dispatcher.h
@@ -17,8 +17,8 @@ namespace system {
 
 // A base class for simple dispatchers. "Simple" means that there's a one-to-one
 // correspondence between handles and dispatchers (see the explanatory comment
-// in core_impl.cc). This class implements the standard waiter-signalling
-// mechanism in that case.
+// in core.cc). This class implements the standard waiter-signalling mechanism
+// in that case.
 class MOJO_SYSTEM_IMPL_EXPORT SimpleDispatcher : public Dispatcher {
  protected:
   SimpleDispatcher();