BUG: read_table raises ValueError when delim_whitespace is set to True
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

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

```python
import io
import pandas as pd

# Build a simple space delimited file buffer to read
f = io.StringIO("a  b  c\n1 -2 -3\n4  5   6")

# This raises an error
df = pd.read_table(f, delim_whitespace=True)

# Setting the "sep" parameter as suggested in the docs works
# df = pd.read_table(f, sep='\s+')
# print(df)

# Not setting the delim_whitespace parameter or setting it to False works, 
#   but without correct formatting.
# df = pd.read_table(f)
# print(df)
```

#### Problem description

If you set the delim_whitespace to True when calling read_table() on a space delimited file, I get the following exception:

```
ValueError: Specified a delimiter with both sep and delim_whitespace=True; you can only specify one.
```

#### Expected Output

I would expect the same output as you get if the sep='\s+' parameter is set (as suggested in the docs).

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

<details>

INSTALLED VERSIONS
------------------
commit           : f2ca0a2665b2d169c97de87b8e778dbed86aea07
python           : 3.6.10.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
Version          : Darwin Kernel Version 18.7.0: Thu Jun 18 20:50:10 PDT 2020; root:xnu-4903.278.43~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.1
numpy            : 1.18.5
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 49.6.0.post20200814
Cython           : None
pytest           : 5.4.3
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.16.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
