Remove DwarfUnit::LabelRange since it's unused.

Seems at some point the intent was to emit fission ranges_base as unique
per CU but the code today emits ranges_base as the start of the ranges
section for all CUs being compiled and all the ranges_base relative
addresses are relative to that. So removing this dead code and leaving
the status quo until there's a reason to change it (perhaps something's
faster if it has distinct ranges for each CU).