git-send-email supports the --in-reply-to option even with
--no-thread. However, the code that adds the relevant mail headers
was guarded by a test for --thread.
Remove the test, so that the user's choice is respected.
Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>