remove holoviews dependency

[chanana]

I don't love it

[jtprince]

Yeah, if the abstraction level of holoviews is not right, then it makes sense to drop it down one notch.

[chanana]

it's really cool as a concept but in practice there's things that become enough of an edge case that we could drop it potentially.

1. you can save to svg if you have a matplotlib or bokeh plot but not a plotly plot. However, plotly shows you how to save the svg separately using the kaleido package.
2. you can set an image size if you're using plotly or bokeh but not if you're using matplotlib. Obviously, matplotlib on its own does have that capability.
3. plotting a scatter plot as Scatter vs Points has subtleties that I need to get acquainted with (in holoviews) and can result in differences in how you interact with the plot for the different engines

These were the things I ran into so far. The other solution here would be to attempt to put PRs upstream in holoviews itself so we can use it more consistently.