Disable ACTS, 1.0 branch (2018.04.16.)
Disabled building ACTS as part of AthenaExternals. Since that package/external really only got into the 1.0 branch by mistake.
As a note to @emoyse and @ivivarel, let's only develop that package in the master branch from now on. It's not a problem to have it in the 1.0 branch, but there's really no reason to keep it up to date in here.