Remove kea, nod, and sms from wmfGetVariantSettings

These languages are now fully supported in core,
so there's no need to mention them here specifically.

Also add a comment that clarifies it for other
maintainers.

Similar patch, for comparison:
I55f2d36aa3c42dc86ee320a7d388138f43f9d346

Bug: T299304
Bug: T296286
Bug: T298075
Bug: T298182
Change-Id: Ib79934b7d0f208ed97c1f59750f75c4f4f4b8a04