Purge serial console after grub is installed

If we do this during install, then configurations written during
finalise.d will overwrite our changes. We need this element to work
so we can address problems with serial console handling in raring
cloud images.

Change-Id: I0880a9609686215732a33c852b41cbc703a72521
Related-bug: #1208912
Merge "Purge serial console after grub is installed"