Bug 1600963 - remove some duplicate includes from ContentPrincipal.cpp; r=mccr8

Depends on D55689

Differential Revision: https://phabricator.services.mozilla.com/D55690

--HG--
extra : moz-landing-system : lando

caps/ContentPrincipal.cpp

@@ -39,9 +39,6 @@
 #include "nsSerializationHelper.h"
 #include "json/json.h"
 
-#include "nsSerializationHelper.h"
-#include "json/json.h"
-
 using namespace mozilla;
 
 static inline ExtensionPolicyService& EPS() {