Clean up release files & downloads

[peterebden]

We have a whole bunch of tarballs and whatnot. I'd like to get rid of them to simplify what we're doing with our downloads.

I think a nice future state might be that the only thing we download is the please binary. On Linux we embed the please_sandbox binary into it, which can be written out somewhere on-demand. (Or we swap entirely to Go sandboxing if that is ready).
The only other binary we ship from this repo is build_langserver which could be downloaded on-demand from plz tool langserver.

We'd need to make sure the update mechanism & the pleasew script are updated to understand this change.