rm dead code for Android 6.0 (M)

assignGrouplessNotifications is only ever needed for Android 7.0 (N) and
higher so bumped this requirement.
Also the Android 6.0 (M) implementation was incomplete, it never
assigned anything from the old notification.

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalNotificationManager.java

@@ -100,15 +100,11 @@ static ArrayList<StatusBarNotification> getActiveGrouplessNotifications(Context
     /**
      * All groupless notifications are assigned the GROUPLESS_SUMMARY_KEY and notify() is called
      */
-    @RequiresApi(api = Build.VERSION_CODES.M)
+    @RequiresApi(api = Build.VERSION_CODES.N)
     static void assignGrouplessNotifications(Context context, ArrayList<StatusBarNotification> grouplessNotifs) {
         for (StatusBarNotification grouplessNotif : grouplessNotifs) {
-            Notification.Builder grouplessNotifBuilder;
-            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
-                grouplessNotifBuilder = Notification.Builder.recoverBuilder(context, grouplessNotif.getNotification());
-            } else {
-                grouplessNotifBuilder = new Notification.Builder(context);
-            }
+            Notification.Builder grouplessNotifBuilder =
+                Notification.Builder.recoverBuilder(context, grouplessNotif.getNotification());
 
             // Recreate the notification but with the groupless key instead
             Notification notif = grouplessNotifBuilder