Remove `simulation::inject_events` (#2265)

This removes `simulation::inject_events` which is made obsolete by
`recipe::event_generators`.
The method exposed internals by giving access to the `lid` of synapses
(over labels) and broke
abstractions (`gid` must be present on this MPI rank and all events must
be deliverable during
the current time (as per documentation)). It was used only in a few
tests and examples and these
uses were replaced by `event_generator`s.