Stop using renv

[davidorme]

We've tried to manage the R environment using renv in the same way that poetry does for Python but:

• It doesn't work in the same way: you don't start a shell that uses the configured environment. Instead you have to activate the environment from within every script. You can do that with a .Rprofile file to source("renv/activate.R"), but that profile file has to be invoked in each directory where code lives and we'd need to get the paths right. This might be automatable, but it isn't a simple system and creates a lot of noise for new users.

• It seems to oddly break in CI for no obvious reason.

We might want to revisit using this, but not right now.