post-receive-email: remove unused variable

prep_for_email neither is passed a fourth argument nor uses it.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/hooks/post-receive-email

@@ -85,7 +85,6 @@ prep_for_email()
 	oldrev=$(git rev-parse $1)
 	newrev=$(git rev-parse $2)
 	refname="$3"
-	maxlines=$4
 
 	# --- Interpret
 	# 0000->1234 (create)