fib_dxr: code hygiene, prune old code, no functional changes

The !DXR2 code corresponds to the original DXR encoding proposal from
2012 with a single direct-lookup stage, which is inferior to the more
recent (DXR2) variant with two-stage trie both in terms of memory
footprint of the lookup structures, and in terms of overall lookup
througput.

I'm axing the old code chunks to (hopefully) somewhat improve readability,
as well as to simplify future maintenance and updates.

MFC after: 1 week

(cherry picked from commit 42b3c16e3051fdc04acc6276dd5d97dbe80ae7eb)