Remove unused functions from Hashtable: consume_high_abund_kmers, count_...

...kmers_within_depth, find_radius_for_volume, count_kmers_on_radius, extract_unique_paths

Fixes issue #599 

- [x] Is it mergable?
- [x] Did it pass the tests?
- [x] If it introduces new functionality in scripts/ is it tested?
  Check for code coverage.
- [x] Is it well formatted? Look at `make pep8`, `make diff_pylint_report`,
  `make cppcheck`, and `make doc` output. Use `make format` and manual fixing as needed.
- [x] Is it documented in the ChangeLog?
- [x] Was a spellchecker run on the source code and documentation after
  changes were made?
Unlike commit messages where we often describe the change in the current tense in change logs we use the past tense to show what has already occurred.