PruneWebHookLogsWorker and RemoveOldWebHookLogsWorker appear to be redundant

Chat thread: https://gitlab.slack.com/archives/C0AR2KW4B/p1539278459000100

RemoveOldWebHookLogsWorker was added in 330789c2. PruneWebHookLogsWorker was later added in https://gitlab.com/gitlab-org/gitlab-ce/commit/75316348c50d03d9aed760e9b603275995e4e3e3.

We should be able to combine them, or deprecate one outright.