Remove PlainTextResponse response_class

[fsiino-nvidia]

https://nvidia.slack.com/archives/C08TG7CLEGY/p1766191655660079

Initially in #290 , the response_class=PlainTextResponse was added to the /global_config_dict_yaml endpoint of the HeadServer as an attempt to debug parsing server info for the ng_status command. This lead to a parsing error in load_from_global_config. This command now uses it's own separate endpoint server_instances, so this needs to be removed.