Remove second call to get_subnets in delete_subnet

delete_subnet made calls to both get_subnet and _get_subnet,
get_subnet uses _get_subnet so multiple database requests were being
made, this patch removes the call to get_subnet and directly calls
_make_subnet_dict using the result of _get_subnet

Change-Id: I9982821dbb9447e35a4b6e50bc5ad31cec417ca5
Closes-Bug: 1366056
Merge "Remove second call to get_subnets in delete_subnet"