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

[MarcoGorelli]

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• I have confirmed this bug exists on the latest version of pandas.

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

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

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()

INSTALLED VERSIONS

commit : 6ad157e
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