Remove event caching code

Flushing the events cache worked by updating a recent number of rows in
the "events" table. This has the result that on PostgreSQL a lot of dead
tuples are produced on a regular basis. This in turn means that
PostgreSQL will spend considerable amounts of time vacuuming this table.
This in turn can lead to an increase of database load.

For GitLab.com we measured the impact of not using events caching and
found no measurable increase in response timings. Meanwhile not flushing
the events cache lead to the "events" table having no more dead tuples
as now rows are only inserted into this table.

As a result of this we are hereby removing events caching as it does not
appear to help and only increases database load.

For more information see the following comment:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6578#note_18864037