Deprecate and remove send_arp_for_ha option

[Ihar Hrachyshka]

It puzzles me why we would want to have it configurable. Having it = 0 is just plain bad (it breaks a floating IP roaming around HA routers), having it = 1 may be unsafe if clients miss the update, having it more than 3 (the default) is probably wasteful. That makes me think that maybe we should not have it in the first place.

The patch that introduced the option also introduced the feature itself, and does not provide any clue around why we would need it: https://review.openstack.org/#/c/12037/

Maybe the option is in the tree because, in Assaf's words, "we're a bunch of lazy developers that like to shift the responsibility to our poor users that have to deal with thousands of configuration options".

I suggest we just move with deprecation and removal here.