Description of expected behavior and the observed behavior

Specifying colors via .hvplot.scatter adds a _color field to hover over, and this seems unintentional.

Complete, minimal, self-contained example code that reproduces the issue

import vega_datasets
import hvplot.pandas

iris = vega_datasets.data.iris()

iris.hvplot.scatter(
    x="sepalWidth",
    y="sepalLength",
    hover_cols=["species"],
    color=iris["species"].map(
        {"setosa": "red", "virginica": "blue", "versicolor": "green"}
    ),
)

Screenshots or screencasts of the bug in action

Version info

hvplot 0.5.2
pandas 0.25.3
bokeh 1.4.0
python 3.7