Revert "neighbour: add support for NUD_PERMANENT proxy entries"

This reverts commit 7c299d0b 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: I2003e9fae1c5c7126918fad5056978ef0a78ed49
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>