Remove apt-xapian-index from ubuntu cloud images

This package recently caused us some very large headaches when it
was updated for a security issue. It is completely unnecessary and
should be removed.

Note that we have recommended that it be removed from the cloud images
in launchpad bug #1227425.

fixes bug #1227420

Change-Id: Ic0d4efa7b44c46271d19576f5191c9421d07c015