Revert "neighbour: add support for NUD_PERMANENT proxy entries"

This reverts commit d427a948 which is
commit c7d78566 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: Ib1d5813633352b2ed811ef9eda97643d239b9414
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>