Remove the abstract flag from CodegenObjectSchema

[karlvr]

I think we should save CodegenObjectSchema for normal implementations. We benefit from the isObject check in templates meaning it's a normal object that you can construct. If we create a CodegenAbstractObjectSchema and a corresponding schema type, then we can generate our abstract objects that we use when making an implementation for an interface, but we won't accidentally try to use them for a purpose for which they're not intended, such as emitting code that does new X().