Remove duplicate git-send-email-perl target in Makefile.
When WITH_SEND_EMAIL was defined, as in the Debian 'deb' target,
git-send-email-perl was added twice to SCRIPT_PERL, leading to a
duplicate definition in the Makefile. Creating a ".deb" then failed.
Signed-off-by: Marco Roeland <marco.roeland@xs4all.nl> Signed-off-by: Junio C Hamano <junkio@cox.net>