Remove duplicated data from legacy events

When event-tracking was fully integrated into the platform, some data was
replicated in the context in the middleware and then later extracted and
moved the old location by event-tracking. The legacy code path was not
updated to remove this transient shim data from the context resulting in
duplication of the information. This patch ensures that the transient
information is removed from the context before emitting the event.

Fixes: AN-2369

@rocha
The idea is that when we decide what do to we remove this line, correct? If so, let's add a TODO comment about it. Otherwise it looks weird that we set the fields and then remove them.