Remove `trust_identity_server_for_password_resets`

The [`trust_identity_server_for_password_resets`](https://github.com/matrix-org/synapse/blob/v1.3.0/docs/sample_config.yaml#L1158-L1169) setting was deprecated in [Synapse 1.4.0](https://github.com/matrix-org/synapse/blob/develop/docs/upgrade.md#upgrading-to-v140), nearly two years ago. We should remove the legacy code which supports that option.