Remove polymorphic discriminator from JSON merge patch tracking

[alzimmermsft]

• Removes polymorphic discriminators from JSON merge patch tracking as they will always be serialized. So, instead of tracking them they have special handling in serialization to always include them.
• Fix where non-nullable properties weren't being checked to be included in JSON merge patch serialization.
• Removed package-private setters from polymorphic JSON merge patch models as they use access helpers instead.