DEPR: fillna 'method'

[jbrockmendel]

Seems like obj.fillna(method="ffill") is redundant with obj.ffill(). If that is accurate, we should deprecate the method keyword in fillna.

[jreback]

+1 this is duplicative

[winedarksea]

-1 this required lots of code changes for no clear benefit to pandas
don't break stuff without clear benefit is my vote