Once Page Previews has been rolled out to enwiki and dewiki, we can:
- Safely remove the instrumentation; and
- Safely (?!) archive the Popups EventLogging tables.
 `ext.popups.core.previewCount` is deleted from localStorage, if it's present.
 The `./counts` module is removed.
 The following properties are removed from the `BOOT` action:
 The `./getBucket` module is removed.
 The `./reducers/eventLogging` module is removed.
 The `./changeListeners/eventLogging` module is removed.
 The `./isEnabled` module is tidied up by removing the `bucket` parameter.
 When can we archive the Popups tables?
 What does the EventLogging instrumentation tell us that the performance instrumentation doesn't?