[SE-0306] Disable actor inheritance.

Actor inheritance was removed in the second revision of SE-0306. Remove
the ability to inherit actors.

Note that this doesn't fully eliminate all vestigates of inheritance
from actors. There are simplifications that need to be performed
still, e.g., there's no need to distinguish
designated/convenience/required initializers. That will follow.

test/ModuleInterface/actor_isolation.swift

@@ -38,9 +38,3 @@ public class C2 { }
 
 // CHECK: @{{(Test.)?}}SomeGlobalActor public class C2
 public class C3: C2 { }
-
-// CHECK: public actor SomeSubActor
-// CHECK-NEXT: @actorIndependent public func maine()
-public actor SomeSubActor: SomeActor {
-  override public func maine() { }
-}