DscpPolicy: remove v6 default route as it is added by the RA

Adding this route before receiving the RA causes the kernel to complain
and fail adding the the global v6 address:
"ICMPv6  : RA: ndisc_router_discovery failed to add default route."

Test: atest DscpPolicyTest
Change-Id: I1d2bfd52e05c334f9808e60455c346b4dca18cf7

tests/cts/net/src/android/net/cts/DscpPolicyTest.kt

@@ -273,7 +273,6 @@ class DscpPolicyTest {
         val lp = LinkProperties().apply {
             addLinkAddress(LinkAddress(LOCAL_IPV4_ADDRESS, IP4_PREFIX_LEN))
             addRoute(RouteInfo(IpPrefix("0.0.0.0/0"), null, null))
-            addRoute(RouteInfo(IpPrefix("::/0"), TEST_ROUTER_IPV6_ADDR))
             setInterfaceName(specifier)
         }
         val config = NetworkAgentConfig.Builder().build()