Removed unnecessary params in cum_func
Picks up from #13167 by properly removing the parameters and ensuring that `numpy` compatibility has been maintained.  The current test suite does a good job of checking that already, so no tests were added.

Closes #13541.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13550?src=pr) is **84.34%**

> Merging [#13550](https://codecov.io/gh/pydata/pandas/pull/13550?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13550   diff @@
==========================================
  Files           138        138          
  Lines         51113      51119     +6   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43109      43115     +6   
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [a01644c...6ba7a77](https://codecov.io/gh/pydata/pandas/compare/a01644c07f376b4e2eb48d8dfca7222473e3088d...6ba7a77b868869a31b26538a9c6dd0d81d71fd8e)
 Looks good to me!
 do need a test for this?
as prior treats didn't catch this
 Catch what?  We already have tests in fact to ensure that the `numpy` call won't break as well as one for non-default arguments in `test_generic.py`.
 what I mean is I suspect we have more functions that have named numpy args - they work but we are specifically naming them 
 AFAIU `cum_func` is a template for **all** of the accumulation functions for `DataFrame` and `Series` at the very least.  What other functions are you referring to?
 no my point is that there might be others that accept the dtype argument (or out) and yet are validated, but we really should remove the actual named arguments. This one was not caught, so there might be others. You might have to inspect them manually. Not sure you can tell the difference between a named arg and a kwarg once your function sees it.
 @jreback : GitHub search indicates that `out=None` is essentially nonexistent in the code-base except for `cum_func`, `algorithms.py` and `function.py` (my `numpy` compat module).

However, the same cannot be said with `dtype=None`.  Manual inspection of every `dtype=None`?  That's just being ridiculous.

Fortunately, a GitHub search indicates that all other instances of `dtype=None` are called from functions that I know definitely do not exist in `numpy`, so I think we are good to go on that.
 ok, then lgtm. yeah the `out` in algorithms.py is ok.
