Revert "ipv6: release nexthop on device removal"

This reverts commit 43e25adc 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: Ibb642b7ce08706067e0d8ae817c636e1525c6bcf
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>