Clean up instructions now that we have pip instructions

[bmw]

certbot/website#698 added pip install instructions to the Certbot site. I think this makes https://certbot.eff.org/docs/install.html#installing-from-source outdated. I think we should probably also consider updating the text at https://github.com/certbot/website/blob/08afdce359d01a6d40a3313f2c96b3b813909084/_scripts/instruction-widget/templates/install/snap.html#L6-L9, however, I think that may not be necessary depending on the updates to https://certbot.eff.org/docs/install.html#installing-from-source.