Remove `simulation::inject_events`

[thorstenhater]

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_generators.

[bcumming]

I vaguely remember adding this in the early days of development, probably pre dating the event generators. It would have been used for unit testing, and is no longer needed.