Remove prepare callback methods from ExitSource
Remove prepare callback methods from ExitSource

Preparation callbacks are not supported for exit sources. See the
sd_event_source_set_prepare() man page.

It is still possible to call the prepare callback methods on an exit
source through a Source &. sd_event_source_set_prepare() will return an
error though and a sd::Exception will be thrown. Programming error.

martin-ejdestig/systemdcpp,martin-ejdestig/systemdcpp