emsdk

Remove `download_even_if_exists` argument to download_and_unzip (#964)

These days this argument really means `install_even_if_directory_exists`
(at least since #9300.

However by the time we call `download_and_unzip` we have already checked
that `is_installed()` is false so we know we want to install for sure.
If the installation directory already existed and contained the correct
contents we would never get as far as `download_and_unzip`.