Raise error in read_csv when arguments header and prefix both are not None
- [x] closes #27394
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

All checks successful.
No, in my view it doesn't need depreciation cycle.

And I have made all the requested changes @simonjayhawkins  > No, it doesn't need depreciation cycle.

IIUC then currently `prefix` is ignored if `header` passed. 

```python
>>> from io import StringIO
>>> s = StringIO("0,1\n2,3")
>>>
>>> import pandas as pd
>>> pd.__version__
'1.0.0rc0+228.g4edcc5541'
>>>
>>> pd.read_csv(s, header=0, prefix="_X")
   0  1
0  2  3
>>>
```

This would now raise a ValueError.

IMO this is a breaking change. see what others think. >IMO this is a breaking change

oh, I think you are right. Hello @rushabh-v! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-02-03 09:25:48 UTC @gfyoung 2 tests are failing, I don't see any obvious connection of them with this PR. 
can you take a look, please? https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=27452&view=logs&j=a3a13ea8-7cf0-5bdb-71bb-6ac8830ae35c&t=add65f64-6c25-5783-8fd6-d9aa1b63d9d4&l=119  Indeed, those test failures look unrelated.  I just restarted the build. >Indeed, those test failures look unrelated. I just restarted the build.

same test failure again. You may need to rebase or merge.  Unclear why these warnings are popping up.

cc @pandas-dev/pandas-core : is there something with `matplotlib` that changed recently? Fails after merge too All greens now @gfyoung  Thanks @rushabh-v !