Code from a message-only proto file includes unused code

Steps to reproduce

1. Create a message.proto file with only "Message" definition. It's intended to be used as common data type definition
2. Create a service.proto file with a "Service" definition that imports from the message.proto file.
3. Generate the code.

Current result

The generated code includes imports "net/http", "github.com/go-chi/chi/v5" and "github.com/go-chi/render" which aren't used.

Example generated code: https://github.com/jvillafanez/prototest001/blob/abf38bb9e4aa6da50ff0c97d59a26ba9905cffd6/gen/ocis/messages/v1/config.pb.web.go

Trying to use / compile the generated code fails.

...../github.com/jvillafanez/prototest001/gen@v0.0.0-20211202132711-abf38bb9e4aa/ocis/messages/v1/accounts.pb.web.go:9:2: imported and not used: "net/http"
...../github.com/jvillafanez/prototest001/gen@v0.0.0-20211202132711-abf38bb9e4aa/ocis/messages/v1/accounts.pb.web.go:11:2: imported and not used: "github.com/go-chi/chi/v5" as chi
...../github.com/jvillafanez/prototest001/gen@v0.0.0-20211202132711-abf38bb9e4aa/ocis/messages/v1/accounts.pb.web.go:12:2: imported and not used: "github.com/go-chi/render"
...../github.com/jvillafanez/prototest001/gen@v0.0.0-20211202132711-abf38bb9e4aa/ocis/messages/v1/config.pb.web.go:9:2: imported and not used: "net/http"
...../github.com/jvillafanez/prototest001/gen@v0.0.0-20211202132711-abf38bb9e4aa/ocis/messages/v1/config.pb.web.go:11:2: imported and not used: "github.com/go-chi/chi/v5" as chi
...../github.com/jvillafanez/prototest001/gen@v0.0.0-20211202132711-abf38bb9e4aa/ocis/messages/v1/config.pb.web.go:12:2: imported and not used: "github.com/go-chi/render"

Start with the message.proto and service.proto reproduction, then inspect the generated .pb.web.go files linked in the report. Generate code for a message-only proto import and compile the result to confirm the unused imports. Done means generated files for this case compile without unused net/http, chi, or render imports.