TST/CLN: remove duplicate data file used in tests (unicode_series.csv)
- [N/A ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ N/A] whatsnew entry

Hello @simonjayhawkins! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/io/conftest.py`](https://github.com/simonjayhawkins/pandas/blob/edc6e9333425b2baaf566c9407acde37ecaa52ec/pandas/tests/io/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/formats/test_format.py`](https://github.com/simonjayhawkins/pandas/blob/edc6e9333425b2baaf566c9407acde37ecaa52ec/pandas/tests/io/formats/test_format.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/parser/common.py`](https://github.com/simonjayhawkins/pandas/blob/edc6e9333425b2baaf566c9407acde37ecaa52ec/pandas/tests/io/parser/common.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/parser/dtypes.py`](https://github.com/simonjayhawkins/pandas/blob/edc6e9333425b2baaf566c9407acde37ecaa52ec/pandas/tests/io/parser/dtypes.py) !

 Can you open an issue for this? Not entirely clear of the context @WillAyd 
> Can you open an issue for this? Not entirely clear of the context

there is no issue. context: just came across duplicate data file and thought it could be de-duplicated!
 Some context: previously, we had a function `get_data_path` that did some black magic to get the directory of the currently running test, so we had to duplicate files for each tree that needed the file :) 

@simonjayhawkins did you notice any other duplicates? thanks @simonjayhawkins 