[nexus] remove active node tracking (#12754)

This commit removes the active node tracking logic from the Nexus
simulation framework.

Previously, `Core` maintained a pointer to the `mActiveNode` and updated
it dynamically during processing and network events. This was necessary
so that log messages could be attributed to the correct node/instance.

With the recent introduction of "instance-aware logging" in the OpenThread
core, the logging mechanism natively knows which `otInstance` generated a
log. Therefore, manually tracking and context-switching the active node
in the Nexus framework is no longer required.

This change simplifies `nexus::Core`, removes context-switching overhead
from heavily utilized inline methods like `Node::Get<Type>()`, and allows
us to simplify the signature of `InfraIf::Receive()`.

tests/nexus/platform/nexus_node.cpp

@@ -166,8 +166,6 @@ void Node::HandleReceive(otMessage *aMessage)
 
     VerifyOrExit(header->GetDestination().GetScope() > Ip6::Address::kRealmLocalScope);
 
-    Core::Get().SetActiveNode(this);
-
     if (header->GetDestination().IsMulticast())
     {
         VerifyOrExit(Get<BackboneRouter::Local>().IsPrimary());