[cert handler] do not observe rel broken directly

lib/charms/observability_libs/v1/cert_handler.py

@@ -344,10 +344,6 @@ def __init__(
             self.certificates.on.all_certificates_invalidated,  # pyright: ignore
             self._on_all_certificates_invalidated,
         )
-        self.framework.observe(
-            self.charm.on[self.certificates_relation_name].relation_broken,  # pyright: ignore
-            self._on_certificates_relation_broken,
-        )
         self.framework.observe(
             self.charm.on.upgrade_charm,  # pyright: ignore
             self._on_upgrade_charm,
@@ -574,14 +570,12 @@ def _on_certificate_invalidated(self, event: CertificateInvalidatedEvent) -> Non
             self.on.cert_changed.emit()  # pyright: ignore
 
     def _on_all_certificates_invalidated(self, _: AllCertificatesInvalidatedEvent) -> None:
-        # Do what you want with this information, probably remove all certificates
-        # Note: assuming "limit: 1" in metadata
-        self._generate_csr(overwrite=True, clear_cert=True)
-        self.on.cert_changed.emit()  # pyright: ignore
-
-    def _on_certificates_relation_broken(self, _: RelationBrokenEvent) -> None:
         """Clear all secrets data when removing the relation."""
+        # The "certificates_relation_broken" event is converted to "all invalidated" custom 
+        # event by the tls-certificates library. Per convention, we let the lib manage the
+        # relation and we do not observe "certificates_relation_broken" directly.
         self.vault.clear()
+        # We do not generate a CSR here because the relation is gone.
         self.on.cert_changed.emit()  # pyright: ignore
 
     def _check_juju_supports_secrets(self) -> bool: