atk: drop ATK_CONF_ENV variables

These are completely historic from the initial commit (810184bb) and
completely unused.

Two separate builds with/without give nothing with:

$ diff -u atk.cv/config.log atk-2.18.0/config.log|grep ^+

And all removals with:

$ diff -u atk.cv/config.log atk-2.18.0/config.log|grep ^-

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>