Inconsistent behaviour on sep/delimiter for pandas.read_csv
Related: #7662

#### Code Sample, a copy-pastable example if possible

```python
from io import StringIO

import pandas as pd


CSV = "a|b\n1|2"
print(pd.read_csv(StringIO(CSV), sep=None, engine='python'))
print('is not the same as')
print(pd.read_csv(StringIO(CSV), delimiter=None, engine='python'))
print('\nand no warning is emitted by')
print(pd.read_csv(StringIO(CSV), sep='|', delimiter=' '))
```
#### Problem description

According to the [documentation](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.read_csv.html),
```
delimiter : str, default None
    Alternative argument name for sep.
```
Thus, I would expect `sep` and `delimiter` to be interchangable. 

#### Expected Output

Specifying `delimiter=None` is equivalent to specifying `sep=None`, and specifying both `sep` and `delimiter` emits a warning or causes an error. Alternatively, either `sep` or `delimiter` should be deprecated.

#### Output of ``pd.show_versions()``

<details>

```
No module named 'dask'

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-45-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL:
LANG: en_DK.UTF-8
LOCALE: en_DK.UTF-8

pandas: 0.24.0.dev0+332.g1f6ddc4
pytest: 3.6.3
pip: 10.0.1
setuptools: 39.0.1
Cython: 0.28.4
numpy: 1.14.5
scipy: 1.1.0
pyarrow: 0.9.0
xarray: 0.10.7
IPython: 6.4.0
sphinx: 1.7.5
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.5
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: 2.5.4
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.5
lxml: 4.2.3
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.9
pymysql: 0.9.2
psycopg2: None
jinja2: 2.10
s3fs: 0.1.5
fastparquet: 0.1.5
pandas_gbq: None
pandas_datareader: None
gcsfs: 0.1.1
```
</details>