Disable sendmail support in KAlarm.

sendmail support is about to be removed from mailtransport. KAlarm has a
lot of code and UI related to it though, pre-dating the introduction of
mailtransport in KDE4. Just removing that is an option, but since the
amount of sendmail-related code suggests there is more rework towards a
pure mailtransport usage needed anyway I just disabled it for now to make
it compile. Please advice on how you want to proceed with this in KAlarm.

CCMAIL: David Jarvie <djarvie@kde.org>