Delete the legacy framework code for manipulating ND offload.

This only worked on broadcom devices, and was superseded in
M by a wifi HAL call made by IpManager.

Test: bullhead builds, boots
Change-Id: I711cae7dafe171c2c8b4e84a229adbcad27f3d14

core/java/android/os/INetworkManagementService.aidl

@@ -103,11 +103,6 @@ interface INetworkManagementService
      */
     void setIPv6AddrGenMode(String iface, int mode);
 
-    /**
-     * Enables or enables IPv6 ND offload.
-     */
-    void setInterfaceIpv6NdOffload(String iface, boolean enable);
-
     /**
      * Add the specified route to the interface.
      */