t4150: remove unused variable

In commit 735285b403 ("am: fix signoff when other trailers are present",
2017-08-08) tests using variable $signoff were rewritten and it is no
longer used, so just remove it from the test setup.

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4150-am.sh

@@ -77,14 +77,12 @@ test_expect_success 'setup: messages' '
 
 	printf "Subject: " >subject-prefix &&
 
-	cat - subject-prefix msg-without-scissors-line >msg-with-scissors-line <<-\EOF &&
+	cat - subject-prefix msg-without-scissors-line >msg-with-scissors-line <<-\EOF
 	This line should not be included in the commit message with --scissors enabled.
 
 	 - - >8 - - remove everything above this line - - >8 - -
 
 	EOF
-
-	signoff="Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>"
 '
 
 test_expect_success setup '