Cleanup installation of root certs on Linux/OSX machines.

We still run this script...
https://github.com/dotnet/wcf/blob/master/src/System.Private.ServiceModel/tools/scripts/InstallRootCertificate.sh

Which attempts to install the root certificate into the **machine root** on various Linus and OXS machines, this script runs before the test build and run process by being inserted as a test command line that runs first via...
https://github.com/dotnet/wcf/blob/master/src/certtest.props

Everything needed by tests should be getting done via our test common infrastructure project but recently due to changes on OSX we no longer have write access to the cert stores from the test user.

So while this script was installing the root cert to the **machine store** we weren't using it anymore, instead we were using our infrastructure common project to install the root cert to the **local user store** which began failing on OSX.

So now it makes sense to use this script as a workaround to the access problem on OSX but we should update the script for this specific reason and not have it install the root cert on any other Linux machines willy-nilly unless specifically needed.