Remove cells code

Thankfully the bulk of this is neatly organized in a single directory
and can be removed, now that the bulk of the references to it have been
removed. The only complicated area is the tests, though effort has been
taken to minimise the diff here wherever possible.

Part of blueprint remove-cells-v1

Change-Id: Ib0e0b708c46e4330e51f8f8fdfbb02d45aaf0f44
Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin