Final deprecation of the basewidget JS in core grades

Within MDL-77991 we moved away from the initial pattern I created within the 4.1 Grade book project.

This was done for a couple of reasons:

1. A backwards compatibility breaking change was introduced in 4.2
2. It is convoluted inside out curried mess (bit awkward considering I made the component..... whoops)
3. The class based system in 4.2 should avoid cases that #1 tried to fix

The current implementors were removed from core as they were replaced with the new class system.

Given the above, We need to now remove core/grades basewidget JS. 

Currently I highly doubt other third part devs have used the component besides Adrian so the debug log in the init should be fine for the time being.

Cheers,