BUG/API : tweaked how AnchoredSizeBar handles font properties

 - removed the explicit `prop` from
    `mpl_toolkits.axes_grid1.anchored_artists.AnchoredSizeBar`
 - pass `fontproperties` through to the base class in place of `prop`
 - if `prop` is not None and `fontproperties` is None, then use
   `prop` in place of `fontproperties`.

close #2394
Woops :wink: