Remove code per spike outcome (T226490#7438685)

- This configuration option is never used in Wikimedia production
- isTalkAllowed content body is merged into isAllowed. Resulting code
is more readable.
- isAllowed('talk') still returns false on main page for anonymous users
which is the current behavior. Even on sv.m.wikipedia.org where talk pages
are shown to anonymous users, we do not show the talk page on the main page.
Add a comment pointing to the associated phabricator ticket

Bug: T226490
Change-Id: I9f34817c5ad654a6a66fe6156850a3e3fee881a7