When a user updates a merge request coming from a fork, they should
not be able to set `force_remove_source_branch` if they cannot push
code to the source project.

Otherwise developers of the target project could remove the source
branch of the source project by setting this flag through the API.

Because we don't have any destroy callbacks (or other logic
triggered on event destroy), there is no reason for deleting events
inefficiently one by one, instead we can use :delete_all.