Yes, they can now be safely deleted using MMC.
As far as I know, MMC doesn't allow you to delete the nodes. It can be done using regedit with the following path: "Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SystemCertificates\avast! SSL Scanner Cache"
As always when using regedit, please do this only at your own risk and don't forget to create a backup first.