CLN: simplify series plotting
Modified these 2 points:
- Simplify `LinePlot._make_ts_plot` to have single flow.
- Simplify `plot_frame` and `plot_series` 's `kind` detection. (Also, setting hexbin default is not necessary because it is done in `HexBinPlot.__init__`)

looks fine and +1 for simplifying the code a little, going to leave it open for a bit for comments (if any) then merge
 thanks!
