chan_iax2: Conditionally remove native support for now.

(issue ASTERISK-21944)


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@396710 f38db490-d61c-443f-a65b-d21fe96a405b
chan_vpb: Effectively remove native support.  Left enough bread crumbs to be able to convert later if needed.

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@396712 f38db490-d61c-443f-a65b-d21fe96a405b