icinga2: Remove unused override file

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

icinga2/systemd.override.conf

@@ -1,4 +0,0 @@
-[Service]
-Environment="ICINGA2_ERROR_LOG=/var/log/icinga2/error.log"
-ExecStart=
-ExecStart=/usr/sbin/icinga2 daemon --close-stdio -e ${ICINGA2_ERROR_LOG}