TST/REF: Remove duplicate .plot.hist() tests, consolidate others
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

When looking into `matplotlib` warnings issued from `test_hist_method.py::TestSeriesPlots::test_hist_legacy` found a bunch of duplicate tests for `hist`. Also moved remaining `hist` tests from `test_series.py` to `test_hist_method.py`.