controller: remove controller grpc service

Remove the controller grpc service along with associated code related to
sessions or remote controllers.

Data types that are still used with complicated dependency chains have
been kept in the same package for a future refactor.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

controller/pb/secrets.go

@@ -5,6 +5,12 @@ import (
 	"github.com/moby/buildkit/session/secrets/secretsprovider"
 )
 
+type Secret struct {
+	ID       string
+	FilePath string
+	Env      string
+}
+
 func CreateSecrets(secrets []*Secret) (session.Attachable, error) {
 	fs := make([]secretsprovider.Source, 0, len(secrets))
 	for _, secret := range secrets {