Fix error raised when router-interface-delete on no-snat routers

Previously, a 500 error would be raised trying to remove a subnet
from a no-snat router because we were trying to remove the snat
rules that didn't exist in the router.

Fixes bug: 1226323

Change-Id: Ieff0ed4bdbac39ac07e597753272a4f5d40cb346
Merge "Fix error raised when router-interface-delete on no-snat routers"