remove legacy_identity_manager runtime flag

Author: kenjenkins

content/docs/reference/runtime-flags.md

@@ -37,7 +37,6 @@ The available flags are:
 | `config_hot_reload` | Enables automatic config reloading triggered whenever a configuration file is written to (either the main Pomerium configuration file or a file referenced from the main configuration). In some rare cases this may not work correctly, so this setting provides a way to disable this behavior. (See issue [#5079](https://github.com/pomerium/pomerium/issues/5079) for more context.) | `true` |
 | `envoy_resource_manager` | Monitors control group (cgroup) memory usage of all processes running in the container (including both Pomerium and Envoy) and applies overload actions when memory thresholds are exceeded to reduce memory consumption. See [memory thresholds](#envoy-resource-manager-memory-thresholds) to review thresholds and their corresponding overload actions. | `true` |
 | `grpc_databroker_keepalive` | _(experimental)_ Enables gRPC keep-alive (HTTP/2 PING) requests on the databroker service connection. This may improve service reliability in [split service mode](/docs/internals/configuration#service-mode) deployments where there are multiple firewalls in the connection path between different Pomerium services. | `false` |
-| `legacy_identity_manager` | The way Pomerium manages IdP session refresh has been newly rewritten in v0.26 for enhanced performance and reliability. When this flag is enabled, Pomerium will revert to the older implementation. | `false` |
 | `match_any_incoming_port` | For a route where the From URL does not contain a port number, allow it to match incoming requests with any port number. See the section on [Port matching behavior](/docs/reference/routes/from#port-matching-behavior) for more details. | `true` |
 | `pomerium_jwt_endpoint` | Temporary opt-out of the `/.pomerium/jwt` deprecation: when set to `true`, Pomerium will continue to issue a JWT from the deprecated `/.pomerium/jwt` endpoint. (This endpoint does not provide the desired security properties for the Pomerium JWT and will be removed in a future release.) | `false` |