t4015: simplify sed command that is not even seen by sed

Noticed by Andreas Schwab; \<LF> inside a double quotes pair is
eaten by the shell to become an empty string and is not doing
anything.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -146,7 +146,6 @@ test_expect_success 'ignore-blank-lines: only new lines' '
 	test_seq 5 >x &&
 	git update-index x &&
 	test_seq 5 | sed "/3/i\\
-\
 " >x &&
 	git diff --ignore-blank-lines >out &&
 	>expect &&