t0018: remove leftover debugging cruft

The actual file is copied out to /tmp, presumably so that the tester can inspect it after the test is done, which may have been a useful debugging aid. But in the final shape of the test suite, such a code should not exist. We cannot even assume that we are allowed to write into /tmp (our TMPDIR may not even be pointing at it) or read from it for that matter. Noticed-by: Randall S. Becker <rsbecker@nexbridge.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>