REF: remove unnecessary SparseDataFrame arith methods
this sits on top of #28027.
Closing to clear the queue.  Will reopen after #28027, which is a blocker for this. Re-opened following #28027. @TomAugspurger did the Sparse-removal get merged?  If so this is now irrelevant. Yep, it's in.

On Wed, Sep 18, 2019 at 9:23 AM jbrockmendel <notifications@github.com>
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> @TomAugspurger <https://github.com/TomAugspurger> did the Sparse-removal
> get merged? If so this is now irrelevant.
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