Revert "ipv6: release nexthop on device removal"

This reverts commit b2f26a27 which is
commit eb02688c upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ied6f94415d8fc58e59fa2bbc15871fb6848cab17
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>