Drop unimplemented envvars from exporter doc

Author: NickCao

docs/source/config.md

@@ -224,15 +224,11 @@ The exporter configuration may also be provided by environment variables which
 may be useful in CI or when writing a script that uses Jumpstarter.
 
 - `JUMPSTARTER_GRPC_INSECURE` - Set to `1` to disable TLS verification.
-- `JMP_EXPORTER_CONFIG` - A path to a exporter configuration YAML file
 to use.
-- `JMP_EXPORTER` - The name of a registered exporter config to use.
 - `JMP_ENDPOINT` - The gRPC endpoint of the Jumpstarter controller server
 (overrides the config value).
 - `JMP_TOKEN` - An exporter auth token generated by the controller
 (overrides the config value).
-- `JMP_DRIVERS` - A comma-separated list of allowed driver namespaces
-to automatically load (overrides the config value).
 
 ## Running an Exporter