Don't check permissions twice in ApiWikibase

Sub-classes are allowed to override getRequiredPermissions. They may
add a permission that's already in the array, e.g. 'edit'. That's
not an error but wastes time by checking the same permission twice.

Change-Id: Iff03571f5115e2f0ce8d983adafbb3a71388c27a
Merge "Don't check permissions twice in ApiWikibase"