systemd does not understand LimitRTTIME=-1. For no limit we have to use
the string 'infinity' (see systemd.exec(5)).
Signed-off-by: Christian Hesse <mail@eworm.de>

This systemd feature has been removed a while ago without replacement,
and it turns out that systemd developers suggest not using control
groups at all to assign real-time privileges.  Therfore, a replacement
feature will not be implement in future systemd releases, and we can
really remove those lines completely.

See http://bugs.musicpd.org/view.php?id=4413

e.g. when running 'update-rc.d mpd disable'

This makes references to mpd(1) and mpd(5) appear in systemd status output.