BUG: don't plot colorbar if c is column containing colors 
- [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.

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

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import matplotlib as mpl
import numpy as np

import pandas as pd

CMAP = "viridis"
df = pd.DataFrame(
    [[5.1, 3.5], [4.9, 3.0], [7.0, 3.2], [6.4, 3.2], [5.9, 3.0]],
    columns=["length", "width"],
)
df["species"] = ['r', 'r', 'g', 'g', 'b']
df.plot.scatter(
    x=0, y=1, c="species", cmap=CMAP,
)
```

#### Problem description

This produces
![image](https://user-images.githubusercontent.com/33491632/82682171-c087f200-9c46-11ea-8636-0c070ec21695.png)

The colorbar to the right of the plot doesn't make any sense here

#### Expected Output
Same plot, but without the colorbar

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

<details>

INSTALLED VERSIONS
------------------
commit           : 6ad157e937624854312934a3e2df8437083cf957
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-51-generic
Version          : #44~18.04.2-Ubuntu SMP Thu Apr 23 14:27:18 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.1.0.dev0+1623.g6ad157e93
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3.post20200325
Cython           : 0.29.16
pytest           : 5.4.1
hypothesis       : 5.8.0
sphinx           : 3.0.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.9.0
bottleneck       : 1.3.2
fastparquet      : 0.3.3
gcsfs            : None
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.4.1
sqlalchemy       : 1.3.16
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : 0.15.1
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.48.0

</details>
