Remove /tmp/downloaded_packages about shiny

[eddelbuettel]

Yes, @csgillespie brought up that very issue here: eddelbuettel/littler#73
We concluded it doesn't necessarily need a fix to install2.r -- there are cases where we want the files to persist -- but an added rm /tmp/downloaded_packages/* may be on order for some use cases.

[cboettig]

@nathanweeks sure thing, adding a bit more cleanup probably makes sense here. happy to take a PR to rocker-org/rocker-versioned and rocker-org/shiny or I'll get around to some more tidy-up once I'm through the start-of-semester chaos 🙃

[nathanweeks]

@eddelbuettel: thanks! I missed eddelbuettel/littler#73 in my search. I agree with its conclusion that conventionalizing cleanup steps specific to Dockerfiles (e.g., rm /tmp/downloaded_packages/* after install2.r) should suffice.
@cboettig : I'll try to find some time in the midst of my own start-of-semester chaos to submit a PR or two, unless you beat me to it (in which case I wouldn't be disappointed!)

[eitsupi]

I think this has been resolved in the latest image, so close it.