Remove 'etc/nova/cells.json'

This was added in change I7046ce55a0a294293c1b1a5fb0f092aeb891ee01 as an
example for how one could configure a cells v1 deployment via a JSON
file. It can be removed now.

Part of blueprint remove-cells-v1

Change-Id: I3235f38c432f1dbafde167ec0e856abe7a38aa8a
Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin