Remove getHasDns() and VirtualNetwork.mHasDns

getHasDns() and VirtualNetwork.mHasDns are no more useful after commit
aops/658122.

Bug: 116539103
Test: system/netd/tests/runtests.sh pass

Change-Id: Ica04984a954a9e89a5eb38a9b262775d686f8ed4
diff --git a/server/NetworkController.h b/server/NetworkController.h
index eb7d670..984f507 100644
--- a/server/NetworkController.h
+++ b/server/NetworkController.h
@@ -105,7 +105,7 @@
 
     int createPhysicalNetwork(unsigned netId, Permission permission) WARN_UNUSED_RESULT;
     int createPhysicalOemNetwork(Permission permission, unsigned *netId) WARN_UNUSED_RESULT;
-    int createVirtualNetwork(unsigned netId, bool hasDns, bool secure) WARN_UNUSED_RESULT;
+    int createVirtualNetwork(unsigned netId, bool secure) WARN_UNUSED_RESULT;
     int destroyNetwork(unsigned netId) WARN_UNUSED_RESULT;
 
     int addInterfaceToNetwork(unsigned netId, const char* interface) WARN_UNUSED_RESULT;