Remove GetDomain functions on assorted method/type/assembly classes which are always in the single app domain

[elinor-fung]

• Remove GetDomain functions on MethodDesc, MethodTable, TypeDesc, and TypeHandle. They always return the single current app domain.
• Remove GetDomain functions and variables on Assembly, DomainAssembly, and Module. The actual domain they are constructed with is always the single current app domain.

[AaronRobinsonMSFT]

There are several places where BaseDomain::IsAppDomain() is called. Ideally BaseDomain would be an abstract class so that is fine, but what about the SystemDomain. Did we confirm that SystemDomain is never cast to a BaseDomain and passed around?

[elinor-fung]

Did we confirm that SystemDomain is never cast to a BaseDomain and passed around?

I checked this for the things I touched.

[jkotas]

Thanks