[BOLT] Remove test for impossible debug ranges condition

Summary:
The condition `DebugRangesOffset == -1U` can never happen since DebugRangesOffset has type `uint64_t` and the value always comes from `RangesSectionWriter->addRanges` which gets its value from `DebugRangesSectionWriter.SectionOffset` which has type `uint32_t`. The condition seems to be left over from a time where something was using `-1` as an error value.

I'm removing that check so I can use `-1` as a tag to refer to the empty range that will be at the beginning of the ranges section.

(cherry picked from FBD18153119)