fig.subplot_mosaic no longer passes the gridspec_kw args to nested gridspecs.

For nested Figure.subplot_mosaic layouts, it is almost always inappropriate for gridspec_kw arguments to be passed to lower nest levels, and these arguments are incompatible with the lower levels in many cases. This dictionary is no longer passed to the inner layouts. Users who need to modify gridspec_kw at multiple levels should use Figure.subfigures to get nesting, and construct the inner layouts with Figure.subplots or Figure.subplot_mosaic.