t4026 (color): remove unneeded and unchained command

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 <redacted>
Acked-by: Jeff King <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>