CLN: remove radd workaround in ops.py
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

Remove workaround added in #353. The test added at that time has been passed.
- https://github.com/pydata/pandas/blob/master/pandas/tests/series/test_operators.py#L1264

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13606?src=pr) is **84.31%**

> Merging [#13606](https://codecov.io/gh/pydata/pandas/pull/13606?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13606   diff @@
==========================================
  Files           138        138          
  Lines         51157      51146    -11   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43131      43122     -9   
+ Misses         8026       8024     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [5701c69...d873aad](https://codecov.io/gh/pydata/pandas/compare/5701c69369264f3aa6f571384602ceec1133dabc...d873aade88fec9e00875cfe9f5d65e87266eb0a1?src=pr)
 lgtm
independent of #13605 ?
 Yes independent, and whichever order is OK.
