Remove _is_precooked pre-cells Zones hacks

pre-Cells Zones used to talk to child zones via the API and thus things
didn't need to be re-serialized for API responses.  The hacks for this
were left around and should be removed.

Fixes bug 1192006

Change-Id: I0b0c7fcad49412659d97037fc3cc8594b46668f1
Merge "Remove _is_precooked pre-cells Zones hacks"