examples/grpc-e2e-tests/tests doesn't build from `go build .`

Issue tracking that `examples/grpc-e2e-tests/tests` seems to not build currently:

```
$ cd examples/grpc-e2e-tests/tests; go build .
client.go:11:2: github.com/GoogleContainerTools/skaffold/examples/grpc-e2e-tests/service@v0.0.0-00010101000000-000000000000: missing go.sum entry; to add it:
go mod download github.com/GoogleContainerTools/skaffold/examples/grpc-e2e-tests/service
$ go mod tidy  1 ✘  1.19.2 
github.com/GoogleContainerTools/skaffold/v2/integration/examples/grpc-e2e-tests/tests imports
github.com/GoogleContainerTools/skaffold/v2/examples/grpc-e2e-tests/service/proto: github.com/GoogleContainerTools/skaffold/examples/grpc-e2e-tests/service@v0.0.0-00010101000000-000000000000: invalid version: unknown revision 000000000000
github.com/GoogleContainerTools/skaffold/v2/integration/examples/grpc-e2e-tests/tests tested by
github.com/GoogleContainerTools/skaffold/v2/integration/examples/grpc-e2e-tests/tests.test imports
github.com/GoogleContainerTools/skaffold/v2/integration/examples/grpc-e2e-tests/service/proto: github.com/GoogleContainerTools/skaffold/examples/grpc-e2e-tests/service@v0.0.0-00010101000000-000000000000: invalid version: unknown revision 000000000000
github.com/GoogleContainerTools/skaffold/v2/integration/examples/grpc-e2e-tests/tests tested by
github.com/GoogleContainerTools/skaffold/v2/integration/examples/grpc-e2e-tests/tests.test imports
github.com/onsi/gomega imports
github.com/onsi/gomega/matchers imports
gopkg.in/yaml.v3 tested by
gopkg.in/yaml.v3.test imports
gopkg.in/check.v1: github.com/GoogleContainerTools/skaffold/examples/grpc-e2e-tests/service@v0.0.0-00010101000000-000000000000: invalid version: unknown revision 000000000000
```

NOTE: This is not a huge issue as running `skaffold build` from examples/grpc-e2e-tests does work as this `tests` dir is not used. As the `tests` dir is not used, it might make sense to delete it outright.

Potential Solutions:
- fix `examples/grpc-e2e-tests/tests` deps, source, etc. so that `go build .` works
- OR remove the `examples/grpc-e2e-tests/tests` as it is unused with respect to skaffold currently