CLN: timeseries in plotting clean up
`format_timedelta_ticks` appears both in `timeseries.py` and `matplotlib.converter.py`, and there is no test. Then i look through the code base, it is only used in `matplotlib.converter.py` for `TimeSeries_TimedeltaFormatter`. So I remove the one in `timeseries.py` and add test for it.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

do you prefer to remove `pos`? @jbrockmendel or parametrize it? fancy a merge? :D  @TomAugspurger  Seems OK to me though not super familiar with this part of the code base. Looking at the comment preceding your change, is _format_coord something we can get rid of as well (OK as separate PR - just curious) lgtm. can you merge master and ping on green. ping @jreback  Thanks @charlesdong1991 