Remove OPENSSL_CONF=/dev/null from tests

Almost two months ago, the warning about non-existing config file was
supressed by setting the environment variable OPENSSL_CONF to /dev/null
everywhere.  Now that this warning is gone, that practice is no longer
needed.

Reviewed-by: Stephen Henson <steve@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>