Tidy up retargeting code and it's debug.log output

Cleans up GetNextWorkRequired() code and debug output:

- remove unused int64_t retargetSpacing
- modulate timespan in an isolated int64_t nModulatedTimespan
- keep nActualTimespan intact, to retain the original value
- compress debug.log output from 5 into 3 lines while retaining
  all information.