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>

Author: gitster

t/t0018-advice.sh

@@ -93,7 +93,6 @@ EOF
 		>README &&
 		GIT_ADVICE=true git status
 	) >actual &&
-	cat actual > /tmp/actual &&
 	test_cmp expect actual
 '