Fix memory leak and crash in Plasma events plugin configuration

Don't leak the internal ETM in PIMCalendarsModel. The Monitor is
already parented to the PIMCalendarsModel, but the ETM is not, so
deleting PIMCalendarsModel destroys the Monitor, but the ETM is left
with dangling pointer to the Monitor.

BUG: 376220
FIXED-IN: 5.6.2