refactor: remove deprecated RepointableActorRef.isTerminated (since Akka 2.2) (#3289)

Motivation:
The public isTerminated method on RepointableActorRef was deprecated since
Akka 2.2 in favor of using context.watch(actor) and receiving Terminated(actor).

Modification:
Remove the deprecated public isTerminated method. The private[pekko] version
from the ActorRef trait remains available for internal use.

Result:
Cleaner public API. Internal callers are unaffected as they use the
private[pekko] ActorRef.isTerminated.

Tests:
Not run - trivial removal of long-deprecated method

References:
None - deprecated API cleanup

Author: He-Pin

actor/src/main/scala/org/apache/pekko/actor/RepointableActorRef.scala

@@ -151,15 +151,14 @@ private[pekko] class RepointableActorRef(
 
   def restart(cause: Throwable): Unit = underlying.restart(cause)
 
+  override private[pekko] def isTerminated: Boolean = underlying.isTerminated
+
   def isStarted: Boolean = underlying match {
     case _: UnstartedCell => false
     case null             => throw new IllegalStateException("isStarted called before initialized")
     case _                => true
   }
 
-  @deprecated("Use context.watch(actor) and receive Terminated(actor)", "Akka 2.2") def isTerminated: Boolean =
-    underlying.isTerminated
-
   def provider: ActorRefProvider = system.provider
 
   def isLocal: Boolean = underlying.isLocal

actor/src/main/scala/org/apache/pekko/routing/RoutedActorRef.scala

@@ -34,7 +34,7 @@ import pekko.dispatch.MessageDispatcher
  * A RoutedActorRef is an ActorRef that has a set of connected ActorRef and it uses a Router to
  * send a message to one (or more) of these actors.
  */
-@nowarn("msg=deprecated")
+@nowarn("msg=BalancingDispatcher.*deprecated")
 private[pekko] class RoutedActorRef(
     _system: ActorSystemImpl,
     _routerProps: Props,

actor/src/main/scala/org/apache/pekko/routing/SmallestMailbox.scala

@@ -92,8 +92,6 @@ class SmallestMailboxRoutingLogic extends RoutingLogic {
     }
   }
 
-  // TODO should we rewrite this not to use isTerminated?
-  @nowarn("msg=deprecated")
   protected def isTerminated(a: Routee): Boolean = a match {
     case ActorRefRoutee(ref) => ref.isTerminated
     case _                   => false