Remove unused code/messages

Remove the following:
* Unused category-based tasks
* GettingStarted.i18n.php shim
* GettingStartedNavbarNoArticle instrumentation [0],[1]
* Anonymous Editor Acquisition controller and tours
* "getting started" revision tagging
* GettingStarted\Hooks::hasEditedMainNamespace method

[0] https://trello.com/c/c45DXjZF/288-updates-for-gettingstartednosuggestedarticle
[1] https://trello.com/c/Um8MWlpa/339-measure-gettingstarted-api-usage

Change-Id: I9ec9b0295acd51ad37f227182f65f47aad3d3c1b
Merge "Remove unused code/messages"