[instance] remove border agent disabling in Finalize (#11711)

This commit removes the explicit call to disable the `BorderAgent`
during instance finalization. This change prevents issues where the
call may trigger platform interactions that can fail due to the
platform layer being deinitialized before the `Finalize` is called.

src/core/instance/instance.cpp

@@ -430,9 +430,6 @@ void Instance::Finalize(void)
     mIsInitialized = false;
 
 #if OPENTHREAD_MTD || OPENTHREAD_FTD
-#if OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE
-    Get<MeshCoP::BorderAgent>().SetEnabled(false);
-#endif
     Get<Mle::Mle>().Stop();
     Get<ThreadNetif>().Down();
     Get<Mac::Mac>().SetEnabled(false);