t4256: don't create unused file

The file "stdout" has been created by the test script since its initial
(and so far only) version added by 3aa4d81f88 (mailinfo: support
format=flowed, 2018-08-25), but has never been used.  Get rid of it.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4256-am-format-flowed.sh

@@ -11,7 +11,7 @@ test_expect_success 'setup' '
 '
 
 test_expect_success 'am with format=flowed' '
-	git am <"$TEST_DIRECTORY/t4256/1/patch" >stdout 2>stderr &&
+	git am <"$TEST_DIRECTORY/t4256/1/patch" 2>stderr &&
 	test_i18ngrep "warning: Patch sent with format=flowed" stderr &&
 	test_cmp "$TEST_DIRECTORY/t4256/1/mailinfo.c" mailinfo.c
 '