Remove lease from Event#reset_project_activity

## What does this MR do?

This removes the exclusive lease used by `Event#reset_project_activity` in favour of conditional UPDATE queries. See dbcc623a901cb3fb725976217416bafad73dbf69 for more information.

## Why was this MR needed?

Obtaining the lease can, for whatever reason, be _really_ slow. See https://gitlab.com/gitlab-org/gitlab-ce/issues/22473 for more information.

https://gitlab.com/gitlab-org/gitlab-ce/issues/22473

See merge request !6678