Trim down to "minilayout"

We don't ever need to checkout all of Chromium OS, most of which we don't use. This helps out buildbot since it doesn't have an option to checkout a limited group and avoid painfully long syncs if anyone forgets -g minilayout with repo init.

Note: the one thing in minilayout this does remove is src/third_party/chromiumos-overlay but we don't need that in the manifest either.