prod_image_util: clean up /etc cleanup code

Pruning files via INSTALL_MASK in the profile is a bit more apropriate
since it allows us to keep most of that info in one place. The only
parts that need to be deleted or adjusted here are inputs and outputs of
`env-update` which has to be run after everything is installed.

Previously we didn't actually clean up `env.d` at all which lead at
least one user to think they should edit those files and run
`env-update` themselves but we don't ship that tool on prod images.

Fixes https://github.com/coreos/bugs/issues/141