Remove unnecessary 'distributed' checks in actor isolation code.
Remove a few places where we are introducing extra distributed-related
actor isolation checking that either isn't necesssary or is incorrect.
Specifically:
* Hopping to a global actor *from* a distributed-actor isolated context
does not go through a distributed thunk (global actors can't have one).
* "Unrestricted" declarations are always unrestricted, so we don't
need an extra distributed check here (it won't ever occur).
* Actor isolation computation doesn't need a special case for
distributed; it marks too much, overriding (e.g.) global actor
isolation.
The primary semantic change visible here is that distributed actors
can now have truly 'nonisolated' members. They aren't required to be
'distributed' because they can't touch state anyway.
