Don't start tethering if IPv6 RPF is not supported.

Bug: 9580643
Bug: 30298058

Change-Id: Ia2e81786527b5718ed7f9dd195a39ea1eb7eb2b4
diff --git a/server/NatController.cpp b/server/NatController.cpp
index 8b4ee11..cda8f5f 100644
--- a/server/NatController.cpp
+++ b/server/NatController.cpp
@@ -376,8 +376,7 @@
         goto err_return;
     }
 
-    // STOPSHIP: Make this an error.
-    if (runCmd(ARRAY_SIZE(cmd4), cmd4) && add && false /* STOPSHIP */) {
+    if (runCmd(ARRAY_SIZE(cmd4), cmd4) && add) {
         rc = -1;
         goto err_rpfilter;
     }