DEPR: SparseDataFrame and SparseSeries subclasses
This is slightly useful in a single dtyped case, but enough of a headache [here](https://github.com/pandas-dev/pandas/issues?utf8=%E2%9C%93&q=is%3Aopen+is%3Aissue+label%3ASparse+DataFrame) that we should simply remove it. Ultimately we should even remove ``SparseSeries`` and just rely on the ``SparseArray`` abstraction and extension types (#19174) .
