Fix memory leak when removing CommandInstance from CommandQueue

The observers for CommandInstance were forcefully cleared in
CommandInstance::DetachFromQueue which caused failure of invoking
CommandInstance::Observer::OnCommandDestroyed() and, as a result,
CloudCommandProxy::OnCommandDestroyed was not called which caused
leaking CloudCommandProxy object instance.

BUG: 25707196
Change-Id: I119a67efce4968ffaa528632e77045265b8e8f2b
Reviewed-on: https://weave-review.googlesource.com/2428
Reviewed-by: Vitaly Buka <vitalybuka@google.com>

src/commands/command_instance.h

diff --git a/src/commands/command_instance.h b/src/commands/command_instance.h
index b1028d0..febe5c5 100644
--- a/src/commands/command_instance.h
+++ b/src/commands/command_instance.h
@@ -89,10 +89,7 @@
 
   // Sets the pointer to queue this command is part of.
   void AttachToQueue(CommandQueue* queue) { queue_ = queue; }
-  void DetachFromQueue() {
-    observers_.Clear();
-    queue_ = nullptr;
-  }
+  void DetachFromQueue() { queue_ = nullptr; }
 
  private:
   // Helper function to update the command status.