Two pass refactoring continued.

Remove testing of whether we estimate that it will be possible
to code an arf at a lower Q than the ambient Q.  This adds quite
a bit of extra code and complexity for marginal gain.

Factored out some code relating to ARNR selection to a separate
function as this is likely to be changed / simplified soon.

Change-Id: Ia1cf060405637ef5bbf7018355437be21d12375f