CLN: consolidate Series.quantile / DataFrame.quantile

[jreback]

these can be moved to core.generic.NDFrame.quantile (leave disabled for ndim > 2 though)
xref #10174

[jbrockmendel]

Series implementation now dispatches to DataFrame. Looking at it, I don't think trying to combine them further would improve clarity. Closable?

[TomAugspurger]

I think so.