refactor kfp-api's on_remove to use KRH built-in delete (#385)

Refactors kfp-api charm's `on_remove()` event handler to use KubernetesResourceHandler.delete() rather than its own removal procedure.  This was done to:
* use the same removal procedure as other charms
* remove the need for the `on_remove()` handler to have a KRH object with full context (in future we will add another Kubernetes object here that requires relation data to render, but the relation data is not guaranteed to be available during remove)

charms/kfp-api/src/charm.py

@@ -11,8 +11,10 @@
 from pathlib import Path
 
 from charmed_kubeflow_chisme.exceptions import ErrorWithStatus, GenericCharmRuntimeError
-from charmed_kubeflow_chisme.kubernetes import KubernetesResourceHandler
-from charmed_kubeflow_chisme.lightkube.batch import delete_many
+from charmed_kubeflow_chisme.kubernetes import (
+    KubernetesResourceHandler,
+    create_charm_default_labels,
+)
 from charmed_kubeflow_chisme.pebble import update_layer
 from charms.data_platform_libs.v0.data_interfaces import DatabaseRequires
 from charms.observability_libs.v1.kubernetes_service_patch import KubernetesServicePatch
@@ -21,6 +23,8 @@
 from lightkube import ApiError
 from lightkube.generic_resource import load_in_cluster_generic_resources
 from lightkube.models.core_v1 import ServicePort
+from lightkube.resources.core_v1 import Service, ServiceAccount
+from lightkube.resources.rbac_authorization_v1 import ClusterRole, ClusterRoleBinding
 from ops.charm import CharmBase
 from ops.main import main
 from ops.model import ActiveStatus, BlockedStatus, MaintenanceStatus, ModelError, WaitingStatus
@@ -142,6 +146,15 @@ def __init__(self, *args):
             ],
         )
 
+    @property
+    def _charm_default_kubernetes_labels(self):
+        """Return default labels for Kubernetes resources."""
+        return create_charm_default_labels(
+            application_name=self.model.app.name,
+            model_name=self.model.name,
+            scope="all-resources",
+        )
+
     @property
     def container(self):
         """Return container."""
@@ -156,6 +169,8 @@ def k8s_resource_handler(self):
                 template_files=K8S_RESOURCE_FILES,
                 context=self._context,
                 logger=self.logger,
+                labels=self._charm_default_kubernetes_labels,
+                resource_types={Service, ServiceAccount, ClusterRole, ClusterRoleBinding},
             )
         load_in_cluster_generic_resources(self._k8s_resource_handler.lightkube_client)
         return self._k8s_resource_handler
@@ -554,14 +569,20 @@ def _on_upgrade(self, _):
     def _on_remove(self, _):
         """Remove all resources."""
         self.unit.status = MaintenanceStatus("Removing K8S resources")
-        k8s_resources_manifests = self.k8s_resource_handler.render_manifests()
-        try:
-            delete_many(self.k8s_resource_handler.lightkube_client, k8s_resources_manifests)
-        except ApiError as error:
-            # do not log/report when resources were not found
-            if error.status.code != 404:
-                self.logger.error(f"Failed to delete K8S resources, with error: {error}")
-                raise error
+        # Define the KubernetesResourceHandler manually because the object storage conflict in
+        # `self._context` is not available anymore, and all we need is the labels/resource_types
+        # to delete the resources anyway
+        k8s_resource_handler = KubernetesResourceHandler(
+            field_manager=self._lightkube_field_manager,
+            template_files=[],
+            context={},
+            logger=self.logger,
+            labels=self._charm_default_kubernetes_labels,
+            resource_types={Service, ServiceAccount, ClusterRole, ClusterRoleBinding},
+        )
+        load_in_cluster_generic_resources(k8s_resource_handler.lightkube_client)
+        k8s_resource_handler.delete()
+
         self.unit.status = MaintenanceStatus("K8S resources removed")
 
     def _on_update_status(self, _):