t4026 (color): remove unneeded and unchained command

Ever since 8b12413 (color: allow multiple attributes 2010-02-27),
diff.color.new has been unused in t4026, so also remove the final unsetting
of that value to make the third to last test pass with appropriate
'&&' chaining.

Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4026-color.sh

@@ -74,7 +74,6 @@ test_expect_success 'extra character after attribute' '
 '
 
 test_expect_success 'unknown color slots are ignored (diff)' '
-	git config --unset diff.color.new
 	git config color.diff.nosuchslotwilleverbedefined white &&
 	git diff --color
 '