Don't build with platform certificate

Change-Id: Iec6798c9139ab62e054efbf714aa07d154f93852
diff --git a/Android.bp b/Android.bp
index fd0ed55..5394330 100644
--- a/Android.bp
+++ b/Android.bp
@@ -96,8 +96,6 @@
         enabled: true,
     },
 
-    certificate: "platform",
-
     sdk_version: "current",
 
     product_specific: true,