This hasn't been used for a while. Since we are going to break Eo a bit anyway
it's a good opportunity to drop this.
This may cause a slight performance issues with legacy events, such as
smart callbacks. This shouldn't really be a problem as we've migrated away from
them. If it does, we need to migrate the remaining parts. Only relevant
for callbacks that are added before the classes are created, which
shouldn't be possible except for smart, only for old evas callbacks.