Fixed #23941 -- Aggregating over decimal field regression

This patch fixes the regression but probably isn't ready for commit just yet. 

What this patch does is ignores the "max_digits" and "decimal_places" arguments of a DecimalField for all annotations. Setting your own arbitrary digits/decimals makes no sense with regards to aggregation, but it may make sense for other annotations.
I'm not sure that this style is appropriate, but it's shorter than 3 lots of "X is not None". Same goes for the sqlite converter.