[Distributed] remove public default AnyActorTransport alias

The previously set public AnyActorTransport was a temporary thing,
and instead we want to set this in every module that declares such
actors.

test/Distributed/distributed_actor_basic.swift

@@ -4,6 +4,9 @@
 
 import _Distributed
 
+/// Use the existential wrapper as the default actor transport.
+typealias DefaultActorTransport = AnyActorTransport
+
 distributed actor DA {
 }