nat46: harden 464xlat teardown logic

nat46: harden 464xlat teardown logic

Quit the teardown function in case the file 464-$cfg-anycast is not
present as it means no clean up of ip rules is required.
It fixes error traces like "cat: can't open '/tmp/464-wan6_4-anycast':
No such file or directory" and "Error: inet6 prefix is expected rather
than "lookup"."
At the same time remove the unused variable anycast_active.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>