BUG: Add Figure.delcolorbar() to fully delete a colorbar

Closes #2688.

This creates a new Figure.delcolorbar() function, named to mimic Figure.delaxes().  The new function will delete a colorbar.  If that colorbar was creating with ```use_gridspec=True```, the function will also reset the gridspec.  I added two tests to confirm that the function works as intended for colorbars creating with ```use_gridspec``` set to either ```True``` or ```False```.