harness: Stop caring about $LINTIAN_BIN_DIR

In the past, it was used to (try to) avoid signature checking, but it
did not work require reliable enough for that.  These days it is just
a different way of pre-appending something to PATH (which you might as
well do from cron or a runner script).

Signed-off-by: Niels Thykier <niels@thykier.net>