remove configuration option 'plat_from_dns64_hostname'

RFC 7050 has been a standard for so long, and is likely used
by so many other implementations, that it doesn't seem to make
sense to use anything else then 'ipv4only.arpa' (also that's
what netd uses, and this code here is really dead, since
in Q we moved the detection out of clatd and into netd).

Test: builds,
  'git grep free_config' comes up empty
  'git grep plat_from_dns64_hostname' comes up empty
Bug: 144730808
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ibf0c8d5e2cec6359fba1f7dd8867bd2742f362b4
3 files changed