upload-pack.c: make send_client_data() return void

upload-pack.c: make send_client_data() return void

The send_client_data() function uses write_or_die() for writing data
which immediately terminates the process on errors. If no such error
occurred, send_client_data() always returned the value that was passed
as third parameter prior to this commit. This value is already known to
the caller in any case, so let's turn send_client_data() into a void
function instead.

Signed-off-by: Lukas Fleischer <lfleischer@lfos.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>