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.
We recently had a change to remove the formatting numbers: 560b4207b1490a7d0cbf70cfbeba7daf2082e5be, if you'd like to follow along.