Field data diet.

We have lots of unused, or almost unused methods in our field data impls,
especially when dealing with ordinals. Let's nuke them.

This removes:
 - `getNumOrds` (same as `getMaxOrd - 1`)
 - `getNumDocs` (unused)
 - gettting ordinals as a `LongsRef`
 - `Ordinals.Docs.ordinals()`