Remove obsolete `digitsTyConKey :: Unique`

This became dead with 1e87c0a6e485e1dbef8e9ed19191e54f6cdc54e0
and was probably just missed.

I plan to re-use the freed up `mkPreludeTyConUnique 23` slot soon
for a new `bigNatTyConKey` (as part of the #9281 effort)